• There is no fgrep command in Perl or File::Find. You have to implement it yourself.
• The "wanted" function (findstuff in your code) is run in the directory where the file resides, not from the top one. The -f test must be run against $_, not $File::Find::name.

Use push to add a member to an array. Here is my attempt to implement what you wanted:

#!/usr/bin/perl
use strict;
use warnings;

use File::Find;


sub fgrep {
    my ($regex, $file) = @_;
    open my $FH, '<', $file or die $!;
    while (<$FH>) {
    return 1 if /$regex/;
    }
    return 0;
}


my @found;
sub findstuff {
    my $file = $_;
    return unless -f $file;
    my $fullpath = $File::Find::name;
    if (fgrep(qr/regex/, $file)) {
    push @found, $fullpath;
    }
}


my @dirs = @ARGV; # Populate @dirs.

find(\&findstuff, @dirs);

print "@found\n";
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Update: push added.