• where is use warnings;?
• do not use uppercase variables names
• also rembember to close your filehandles anyway: it is safer.
• chop is not chomp
• Avoid a or b as variable name: the scalar form are special variables and even if the hash is not, avoid it anyway. Instead choos meaningfull variables names
• when learning or debugging i think is preferable to write down plain syntaxes: you have a superfluous bare block: why? the syntax inside it is not so begenner's one. How can inpsect it without a place where insert the basic debugging tool aka print?

#{
#  print "$_\n" for grep {$_} @a{keys %b};
#}
#
# should be something like (untested..)

foreach my $bkey (keys %b) {
    warn "key not defined" unless $bkey; ## what is the purpose of you
+r "grep {$_}"???
    if ($a{$bkey}){print "FOUND: [$bkey] in the hash \%a\n"}
    else{print "NOT found key [$bkey] in the hash \%a\n"}
}
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