Looking at the source, it looks like it happens in Catalyst::Engine::prepare_uploads, which reads

sub prepare_uploads {
    my ( $self, $c ) = @_;

    my $request = $c->request;
    return unless $request->_body;

    my $uploads = $request->_body->upload;
    my $parameters = $request->parameters;
    foreach my $name (keys %$uploads) {
        my $files = $uploads->{$name};
        my @uploads;
        for my $upload (ref $files eq 'ARRAY' ? @$files : ($files)) {
            my $headers = HTTP::Headers->new( %{ $upload->{headers} } 
+);
            my $u = Catalyst::Request::Upload->new
              (
               size => $upload->{size},
               type => $headers->content_type,
               headers => $headers,
               tempname => $upload->{tempname},
               filename => $upload->{filename},
              );
            push @uploads, $u;
        }
        $request->uploads->{$name} = @uploads > 1 ? \@uploads : $uploa
+ds[0];

        # support access to the filename as a normal param
        my @filenames = map { $_->{filename} } @uploads;
        # append, if there's already params with this name
        if (exists $parameters->{$name}) {
            if (ref $parameters->{$name} eq 'ARRAY') {
                push @{ $parameters->{$name} }, @filenames;
            }
            else {
                $parameters->{$name} = [ $parameters->{$name}, @filena
+mes ];
            }
        }
        else {
            $parameters->{$name} = @filenames > 1 ? \@filenames : $fil
+enames[0];
        }
    }
}
[download]

Looks like

$headers->content_type
[download]

can return more than one value. Replace it with

scalar($headers->content_type)
[download]

Looking at the source, it looks like that happens in Catalyst::Engine::prepare_uploads

sub prepare_uploads {
    my ( $self, $c ) = @_;

    my $request = $c->request;
    return unless $request->_body;

    my $uploads = $request->_body->upload;
    my $parameters = $request->parameters;
    foreach my $name (keys %$uploads) {
        my $files = $uploads->{$name};
        my @uploads;
        for my $upload (ref $files eq 'ARRAY' ? @$files : ($files)) {
            my $headers = HTTP::Headers->new( %{ $upload->{headers} } 
+);
            my $u = Catalyst::Request::Upload->new
              (
               size => $upload->{size},
               type => $headers->content_type,
               headers => $headers,
               tempname => $upload->{tempname},
               filename => $upload->{filename},
              );
            push @uploads, $u;
        }
        $request->uploads->{$name} = @uploads > 1 ? \@uploads : $uploa
+ds[0];

        # support access to the filename as a normal param
        my @filenames = map { $_->{filename} } @uploads;
        # append, if there's already params with this name
        if (exists $parameters->{$name}) {
            if (ref $parameters->{$name} eq 'ARRAY') {
                push @{ $parameters->{$name} }, @filenames;
            }
            else {
                $parameters->{$name} = [ $parameters->{$name}, @filena
+mes ];
            }
        }
        else {
            $parameters->{$name} = @filenames > 1 ? \@filenames : $fil
+enames[0];
        }
    }
}
[download]

[The indexing is messed up for this thread. My earlier reply isn't showing, so I'm reposting it]

Looks like

$headers->content_type
[download]

can return more than one value. Replace it with

scalar($headers->content_type)
[download]

you're right. amending catalyst::engine to say

            my $u = Catalyst::Request::Upload->new
              (
               size => $upload->{size},
               type => $headers->content_type,
               headers => $headers,
               tempname => $upload->{tempname},
               filename => $upload->{filename},
              );

                                                        use Data::Dump
+er;
                                                        die '$headers:
+'.Dumper($headers).
                                                            'type:'.Du
+mper($headers->content_type).
                                                            '$u:'.Dump
+er($u).
                                                            "\nek!";
[download]

gives

[error] Caught exception in engine "$headers:$VAR1 = bless( {
                 'content-disposition' => 'form-data; name="SourceFile
+_1"; filename="aaa.jpg"',
                 'content-type' => 'image/jpeg; charset=utf-8; Content
+-Transfer-Encoding: binary'
               }, 'HTTP::Headers' );
type:$VAR1 = 'image/jpeg';
$VAR2 = 'charset=utf-8; Content-Transfer-Encoding: binary';
$u:$VAR1 = bless( {
                 'aaa.jpg' => undef,
                 '/tmp/P3v6fH2JR6' => 'filename',
                 'HTTP::Headers=HASH(0x40e9e58)' => 'tempname',
                 'type' => 'image/jpeg',
                 'charset=utf-8; Content-Transfer-Encoding: binary' =>
+ 'headers',
                 'size' => 374205
               }, 'Catalyst::Request::Upload' );

ek! at /Library/Perl/5.8.6/Catalyst/Engine.pm line 556."
[download]

so it looks like $headers->content_type is indeed returning two values. I'll suggest a patch.