It is not clear what you are asking.

For information about perl hashes, please read the "perldata" documentation.
It seems like you would like to construct a hash with keywords, so you can reference it like :

my %kwhash = (word1=> undef, word2=>undef);
# SPLIT your line of input, then match each word
while (my $line = <SIDX2>){
  for my $word (split / /, $line){
    if (defined $kwhash{$word}){
       # Process for FOUND keyword
        
    }
  
}
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