As a matter of fact, read the whole book online for free! It's very informative. At Ovid's Beginning Perl

And you can write your own function that takes an implicit $_ using the (_) prototype.

My favourite use for $_ is when defining a function that accepts a coderef. Example

# This is a fairly useless function which is basically
# just a copy of grep which enforces numeric context
# on the list items...
#
sub filter_nums (&@)
{
  my $coderef = shift;
  return
    grep { $coderef->() }
    map  { 0 + $_ }
    @_
}

my @even = filter_nums { not($_ % 2) } qw(1 2 3 4 potato);
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perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

Most of the time, you use $_ without actually using it -- without typing it, that is. Take this code:

my @array;
while(<$file>){
  chomp;
  tr/A-Z//d;
  push @array, [split /\s+/];
}
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Each of those lines of code operates on $_, though you don't see the variable explicitly typed at all. That's because many Perl functions operate on $_ by default if they aren't given a specific argument.

Some say using $_ is bad practice and makes code less maintainable. That may be true for longer loops containing many other variables, but for short loops like the above, in my opinion it's actually clearer than:

my @array;
while(my $line = <$file>){
  chomp $line;
  $line =~ tr/A-Z//d;
  push @array, [split /\s+/, $line];
}
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