What is that?

There are a couple of tricks to help you answer that. Since $_ has been defined, start with that. Using perldoc, just enter

perldoc -v '$_'
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It'll return the info for $_. Note that you'll need to use single-quotes around $_ to avoid interpolation.

$_ is a perl builtin variable, but how would you know that? You can get a complete list of builtin variables by using B::Keywords:

#!/usr/bin/perl

use strict;
use warnings;
use B::Keywords qw(:all); 

print join "\n", @Symbols, "\n";
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What if you encounter a function that you don't know? Using perldoc, enter, for example, the function "my":

perldoc -f my
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To get a categorized list of functions, you'll use Pod::Functions. You'll need to enter the complete path to your Pod::Functions:

perl /usr/lib/perl5/5.8.8/Pod/Functions.pm
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I'm not aware of a quick and easy way to get perl's operators.

Update: ELISHEVA has reminded me that you can access perlop by entering doc://perldoc in the perlmonk's search bar.