#!perl -w
use strict;
my %hashola = (fish => "test" );

sub f($){ return $hashola{shift @_};  }

my $rock = "this is a #fish#\n";
my $block = $rock;

print $rock;

$block =~ s/#(\w+)#/&f($1);/; # expands $1, doesn't call &f

print $block;

$block = $rock; # restoree

$block =~ s/#(\w+)#/$hashola{$1}/; # expands the variable

print $block;

$block = $rock; # restoree

$block =~ s/#(\w+)#/&f($1)/e; # calls the function

print $block;

$block = $rock; # restoree

$block =~ s/#(\w+)#/$hashola{$1}/e;

print $block;

$block = $rock; # restoree

__END__
F:\dev>perl f
this is a #fish#
this is a &f(fish);
this is a test
this is a test
this is a test

F:\dev>