use strict;
use warnings;
use Text::Balanced qw(extract_bracketed);

my $code = "sub blah {\n {}\n {}\n {}\n {}\n}
and some other stuff
sub another {\n}";

for my $end (find_ends($code)) {
    $code = substr($code, 0, $end)
          . "}"
          . substr($code, $end);
}
$code =~ s/sub/{sub/g;

print "$code\n";

sub find_ends {
    local $_ = $_[0];

    my @ends;
    while (/(sub\s+\S+\s*)(?={)/g) {
        () = extract_bracketed($_, '{}');
        push @ends, pos;
    }

    return @ends;
}