DISCLAIMER: I haven't ever used Net::FTP.

You can open scalars (v5.8.0 required) by passing a reference to that scalar to open:

my $pseudo_file = "line1\nline2\n";
open my $rfd, '<', \$pseudo_file or die "open1: $!";
open my $wfd, '>', 'foo' or die "open2: $!";
print $wfd $_ while <$rfd>;      ## or 'print $wfd (<$rfd>);' if file 
+isn't large
close $wfd;
close $rfd;
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So, according to the Net::FTP doc, I'd change your code to:

open my $fh, '<', \$pseudo_file_already_generated or die "open: $!";
$ftp->login("perl",'monk') or die "blah";
$ftp->put($fh, 'fh.txt') or warn "blah";
$ftp->quit;
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Update: Better english.