$word = shift;
%sent = (
  one => "This is sentence one. I wonder what the word is.",
  two => "This would be sentence two.",
  thr => "Is this sentence three. I think it is.  I sure hope it is.",
);

@matches = grep $sent{$_} =~ /\b$word\b/, keys %sent;

print "The matching sentences are:\n";
for (@matches) {     
    print; print " => ", $sent{$_}, "\n";
}