The mkdir function creates directories (which I assume are what you mean by "subfolders"). The open function can open files even if you prepend a path to them. If you want to create multiple directories in one go, have a look at File::Path.

use strict;
use warnings;

my $outdir = "output";

if (! -d $outdir) {
    warn "Creating '$outdir'";
    mkdir $outdir
        or die "Couldn't create '$outdir': $!";
};

my $outname = "$outdir/test.txt";
open my $outfile, ">", $outname
    or die "Couldn't create '$outname' : $!";
print $outfile, "Hello World\n";
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