$ perl -E 'my @x = qw{b c a}; say scalar grep { /^(?:a|b|c)$/ } @x'
3
$ perl -E 'my @x = qw{x y z}; say scalar grep { /^(?:a|b|c)$/ } @x'
0
$ perl -E 'my @x = qw{x c y a b z}; say scalar grep { /^(?:a|b|c)$/ } @x'
3