In general, most questions you might have about regular expressions can be found in perlre or perlretut.

In regular expressions, square brackets indicate character classes. In this case you will match on any of the following list of characters:

[ \ s ( /

I'm actually a little surprised that the regular expression engine doesn't throw an error on an unescaped open square bracket in a character class, but it seems to have no problem parsing it, so...

The rest of the expression just requires the literal string ssshd\s. Thus the following outputs 1:

print '(ssshd\s' =~ m<[[\\s(/]ssshd\\s>;