Start by making sure the subfolder is there (use mkdir).

my $subdir = 'subfolder';
if ( ! mkdir( $subdir ) && ! -d $subdir ) {
    die "Can't create '$subdir': $!";
}
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Then just open the file you want to write into.

my $outfile = "$subdir/file.txt";
open my $out_fh, '>', $outfile
    or die "Can't write '$outfile': $!";
print $out_fh $source;
close $out_fh or die "close() failed: $!";
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Note that I'm using a lexical filehandle instead of a global one, and I'm using the three-argument form of open. Also, the directory/file that we make is relative to current directory when the program is executed, not to where the program actually lives. For the latter, have a look at $0.