The code is testing whether 9eaf is a substring of the value of $tag.
$tag =~ /9eaf/
is short for
$tag =~ m/9eaf/
where m// is the match operator. It matches the regular expression pattern (regexp) 9eaf against the value bound by =~ (returned by the left hand side of =~).
Operators, including m// and =~, are documented in perlop.
Regular expressions (e.g. 9eaf) are documented in perlre, perlretut.
