Yes, in such a case, you want to declare the array before entering the loop. That's what choroba is doing in the suggested code, declaring @found right before entering the loop:

my @found;
sub findstuff {
    my $file = $_;
    return unless -f $file;
    my $fullpath = $File::Find::name;
    if (fgrep(qr/regex/, $file)) {
    push @found, $fullpath;
    }
}
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So, to push to the array with the array inside the foreach block, I would do this:

my @array1;
my @array2;

foreach (@array1) {
    undef @array2;
    find(\&mysub, cwd);
    ..do stuff with @array2
}

sub mysub {
        my $file = $File::Find::name;
        ..filter stuff here...
        push(@array2, $file);
}
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Or is there a nore elegant way of doing this?

Why do you undef @array2 within your foreach loop? Do you need a fresh xopoty of @array2 each time? Probably not.