Maybe the point your teacher is trying to make is to investigate some of the Fine Modules on CPAN. E.g.:

>perl -wMstrict -le
"my @rray1 = qw/boy  boymonkey  boydog/;
 my @rray2 = qw/girl girlmonkey girldog/;
 ;;
 use List::MoreUtils qw(pairwise);
 use vars qw($a $b);
 my @unions = pairwise { [ $a, $b ] } @rray1, @rray2;
 ;;
 use Data::Dumper;
 print Dumper \@unions;
"
$VAR1 = [
          [
            'boy',
            'girl'
          ],
          [
            'boymonkey',
            'girlmonkey'
          ],
          [
            'boydog',
            'girldog'
          ]
        ];
[download]

Update: Although if you're supposed to do it with a single for-loop and no indexing, maybe your teacher has something like this in mind (but this destroys the  @gals array):

>perl -wMstrict -le
"my @guys = qw/boy  boymonkey  boydog/;
 my @gals = qw/girl girlmonkey girldog/;
 ;;
 my @unions;
 for my $guy (@guys) {
   push @unions, [ $guy, shift @gals ];
   }
 ;;
 use Data::Dumper;
 print Dumper \@unions;
"
$VAR1 = [
          [
            'boy',
            'girl'
          ],
          [
            'boymonkey',
            'girlmonkey'
          ],
          [
            'boydog',
            'girldog'
          ]
        ];
[download]