I've done somthing similar before myself. I think you want something like this:

use strict;
use File::Find;

# Perl script to search files for particular strings.
# Recurses into subdirectories.
# If you don't specify a directory to search on the command line,
# it will default to the current directory.

# define strings to look for here:
my @Constants = (
  "foo",
  "bar",
  "baz",
);
  
my $path = $ARGV[0] || ".";
my $count=0;

find( sub {
  my $n = $File::Find::name;
  # search for files ending in .h or .c
  if ($n =~ /\.h$/ || $n =~ /\.c$/ ) {
    ++$count;
    # might want to use die instead of warn
    open(IN, "$n") or warn "can't open file $n : $! ";  
    while (<IN>) {
      my $line = $_;
      for my $search_string (@Constants) {
        # the "i" makes the match case insensitive
        if ($line =~ /$search_string/i) {      
          # print filename and line number and matching string
          print "$n : $. : $search_string\n";           
        }
      }
    }
    close (IN);
  }
}, $path);


print "Found matches in $count files.\n";
[download]

update: Fixed typo in original post - the path should default to the current working directory. Also, looking at Grandfather's reply, if you want a pattern like "foo" to NOT match something like "food" or "foobar", change the pattern match line to this:

if ($line =~ /\b$search_string\b/i) {
[download]

This adds "end of word boundary" around the pattern.