grep in Perl is essentially a simple method for filtering a list. I tend to recommend avoiding it while you are still familiarizing yourself with Perl's constructs.

grep allows you to accomplish something like:

my @second;
for my $element (@first) {
    push @second, $element if condition($element);
}
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in a concise syntax. Simple filtering by a regular expression is a classic use case. If you have side effects or complex tests, then grep may be a bad choice.