You could make it look better (but probably less efficient, although you should benchmark this) by using a hash for the substitution.

my %subst = ( 'AAA' => 'EEE', 'CCC' => 'NEWWORD',
              'UUU' => 'NEW', 'ERD' => 'IPK' );
my $pattern = join('|', keys %subst);
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and replacing the body of the foreach by:

  $line =~ s/($pattern)/$subst{$1}/gi;
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Note: this is untested.

Hope this helps, -gjb-

Update: thanks to broquaint for pointing out that I should drop the /e as regex modifier. Silly, I wasn't thinking.