use strict;
use warnings;

my @rec = (
    'N(8) -- H(15) .. O(9)',
    'N(8) -- H(15) .. N(8)',
    'N(8) -- H(16) .. O(9)',
);

my %count;
for (@rec) {
    /(H\(\d+\))/ && do { push @{$count{$1}}, $_ }
}

for (keys %count) {
    if ($#{$count{$_}}) {
        print "$_\n" for @{$count{$_}}
    }
}

__END__

# another go...

$,="\n";
for (keys %count) {
    print @{$count{$_}} if $#{$count{$_}}
}