# for loop
for my $i (0 .. $#a1) {
    for my $j (0 .. $#a2) {
        push @indices, $i
            if $a2[$j] eq $a1[$i];
    }
}

##</code><code>##

# schwartzian [sp?] transform (perhaps not a paradigmatic example though)
my @indicies = map {$_->[0]}
    grep {$a1[$_->[0]] eq $a2[$_->[1]]}
    map { my $l = $_; (map {[$l,$_]} 0 .. $#a2) } 0 .. $#a2;