my $subdir = 'subfolder';
if ( ! mkdir( $subdir ) && ! -d $subdir ) {
    die "Can't create '$subdir': $!";
}

##</code><code>##

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: $!";