#!/usr/bin/perl -w
use strict; 

my %acro = (
    HTML   => "Hypertext Markup Language",
    ICBM   => "Intercontinental Ballistic Missile",
    EEPROM => "Electronically-erasable programmable read only memory",
    SCUBA  => "Self Contained Underwater Breathing Aparatus",
    FAQ    => "Frequently Asked Questions",
    LCARS  => "Library Computer And Retrieval System",
    NASA   => "National Aeronautical and Space Administration",
);

my $text=<<TEXT;
Here is a text that includes LOTS of accronyms like HTML, 
ICBM, SCUBA etc. Maybe this should be a FAQ.
TEXT

# substitute all accronymns in the text
$text=~ s{([A-Z0-9]+)}                 # find all upper-case words (stored in $1)
         { $acro{$1}                   # is it in %acro?
                    ? "$1 ($acro{$1})" # yes, expand it
                    : $1               # no, leave it as is
         }gex;                         # g means to do it as many times as possible
                                       # e means to execute the code in the replacement part
                                       # x allows multi-line regexp and comments

print $text;