It will be true if $input represents a pattern that will be a positive match against the string held in $_

The /o modifier has fallen out of favor nowadays, largely replaced by the qr// operator. But in this case what it means is that once the RE is compiled once, it won't be recompiled again, even if the value of $input changes. The choice of regexp delimiters as the '|' character seems a little ackward, as it makes alternation difficult.

perlre, perlrequick and perlretut can be helpful.