my $existingdir = './mydirectory';
mkdir $existingdir unless -d $existingdir; # Check if dir exists. If not create it.
open my $fileHandle, ">>", "$existingdir/filetocreate.txt" or die "Can't open '$existingdir/filetocreate.txt'\n";
print $fileHandle "FooBar!\n";
close $fileHandle;
This should work for you.