use strict;
use warnings;

$|++;

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",
);

INPUT: {
    print "Please enter an acronym: ";
    chomp( $_ = <STDIN> );
    $_ = uc or redo INPUT;

    last INPUT if /^Q/;

    unless (exists $acro{$_})
    {
        print "I don't know what '$_' means.\n";
        redo INPUT;
    }

    print "$_ ($acro{$_})\n";
    redo INPUT;
}
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Some explanations:

1. Always begin programs with the top two lines. It's easier to have the compiler do the bookkeeping for you.
2. The $|++ is to disable output buffering. (q.v. Suffering from Buffering for more info.)
3. Then, I set up the lookup table for the acronyms.
4. Now, here's the goofy thing. You'll notice I'm using a named block and using last and redo. (I can't use next cause I'm not in a loop, but redo does something similar.) Sometimes, a certain construct makes a given algorithm easier to express for a given person. I like using named blocks and redo to handle keyboard input. Others like while-loops, goto, and all sorts of other ideas. I like named blocks.
5. Now, I assign to $_, which has the fancy name of "local topic". This is the default variable for many functions, including regexes.
6. I want to compare case-insensitively, so I uc the string. (uc defaults to using $_.) Now, if there is nothing, uc will return the empty string. This allows me to use logic short-circuits to redo the loop if nothing was entered. (Try it!)
7. I allow for a quit scenario by stopping if anything that begins with 'Q' or 'q' is entered. (You might want to change this to allow acronyms that begin with 'Q'.)
8. I use the exists function to have the hash do the lookup for me (instead of me coding a for loop). This also has the benefit of keeping my hash clean. If it doesn't exist, print a message and redo the block.
9. If it does exist, print the acronym and definition, then redo the block.

If you have any questions, please don't hesitate to ask!