I'd put the input into a hash; then scan nouns.txt (once) and for each word, see if it is in the hash.

use strict;
use warnings;

my %words = map { s/[.?!]$//;
                  lc $_, $_ 
                } 
            split/\s+/,<STDIN>;

open(NOUNS,'<',"nouns.txt") or die "Can't open noun database: $!\n";

while(<NOUNS>){
    s/\s+//g;
    print "\n!MATCH! ~ $words{lc $_} is a noun\n"
        if exists $words{lc $_};
}

close(NOUNS);
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