In this case I often use an alternate method, especially if there are lots of accronyms in the list: I scan the text for things that look like accronyms, and see if they exist in the hash:

#!/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 (st
+ored 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 time
+s as possible
                                       # e means to execute the code i
+n the replacement part
                                       # x allows multi-line regexp an
+d comments

print $text;
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Note that if accronyms can include lower case letters, you will need to change the matching regexp to include them.