See the section Delayed Response and Streaming Body in the PSGI specification.

What you want is something like this (untested):

use strict;
use warnings;
####use Plack::Response; # doesn't seem to support streaming response

use Path::Tiny;
use File::MimeInfo;

sub return_psgi {
    my $self = shift;
    my $path_file = shift;

    my $content_type = mimetype($path_file);
    $content_type = 'application/octet-stream' if not defined $content
+_type;
    
    my $content = path($path_file);
    my $content_download = $content->slurp;

    require File::Spec::Unix;
    my $basename = $path_file;
    $basename =~ s|\\|/|g;
    $basename = ( File::Spec::Unix->splitpath($basename) )[2];
    $basename =~ s|[^\w\.-]+|_|g;

    return sub {
       my $responder = shift;
        my $writer = $responder->(
            [ 200, [
                   'Content-Type' => $content_type,
                   'Content-Disposition' => qq[attachment; filename="$
+basename"],
               ]
            ]
        );

        open (FILE, "< $path_file") or die "can't open $path_file: $!"
+;
        binmode FILE;
        local $/ = \102400;
        while (<FILE>) {
            $writer->write($_);
        }
        $writer->close();
        close FILE;
    }
}
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