Further to Laurent_R's post: This is known as "autovivification" (see perlglossary) and is occurring with all the variables in the OPed code except $a and $b, which are pre-existing Perl special variables (see perlvar). Only package-global variables are created in this way, so this practice is officially Frowned Upon and is precluded by using strictures (see strict). In almost all cases, it's better to pre-declare lexical variables with my.

... what do the pounds in $t#$t#$t do?

They match against literal  '#' characters in a string. They have no special function in a regex unless the  /x regex modifier is used (see Modifiers), in which case  # becomes a comment metacharacter/operator and must be escaped in some way to match against a literal character.