perlintro doesn't mention a then belonging to an if, because Perl is not Visual Basic.

Just write

if (defined(param('foo'))) {
   print "yes, we have a foo\n";
} else {
   print "out of luck, pal\n";
}
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I also don't understand what you mean that I don't have to compare booleans to true or false.

The question is $thing the same as true? is nearly the same as is $thing true?. When you write if ($thing) { ... } then perl already asks is $thing true? - no need to make it more complicated by explcitly comparing it to true (which doesn't exist in Perl 5 anyway).