Because File::Find's "wanted function" does not return the file names, the array that will hold the matching files needs to be in scope, e.g. a package or global variable.

Note: File::Find's find function takes an array as its 2nd parameter, so foreach (@dirs) isn't needed.

use strict;
use warnings;
use File::Find;

my @files;
my @dirs = qw(/some /list /of /dirs);

sub findstuff {
   my $file = $File::Find::name;
   return unless -f "$file";
   if (grep { /regex/ } "$file") {
      print "Found regex in $file\n";
      push @files, $file;
   }
}

find(\&findstuff, @dirs);
print "Results:\n@files\n";
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Update: Interesting: I interpreted the OP's if (fgrep { /regex/ } "$file") as a typo (fgrep vs grep) for a check for a regex in the filename. choroba's response below makes better sense of that.