You don't need the extra {}s around frq_node in the if block... $frq_node{$key}[0]{$to}++ should work.

In general, I think it's easier to use a reference to an anonymous hash in complicated data structures like these.

Depending on your data, you might be able to get away with two ifs instead of the if/elsif block.

Also, you didn't close the elsif pattern match.

my $frq_node = {};

while (<FH>){
    $frq_node->{$key}[0]{$to}++ if /sending ping/;
    $frq_node->{$key}[1]++      if /recevied ping response/;
}
[download]

might work for you.