The foreach loop aliases each element of the array to a loop variable for each iteration. If you don't include a loop variable explicitly, perl uses $_ by default.

The problem with your original code is that each iteration of the loop replaces the contents of your @wordlwr array with the current contents of $_ instead of appending $_ to the array as you expect. I think that the following code is what you want.

use strict;

my @words = qw/ONE TWO THREE/;
my @wordlwr = ();

foreach (@words) {
    push @wordlwr, lc($_);
}

foreach (@wordlwr) {
    print "$_\n";
}
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