I'd probably write it as:

sub arrayindex {
    my $value = shift;

    for (0..$#_) {
        return $_ if $_[$_] eq $value;
    }

    return -1;
}
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---

Update code changed to reflect MeowChow's comment.

The wrong way to do it would be:

my $index;
for $index (0..$#values) {
    last if $values[$index] eq $value;
}
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[ ar0n ]