Hello neszt76 and welcome to PerlMonks. We use Perl here.

perl -nE '$f.=$_}{$t{$_}=()=$f=~/\Q$_/g for $f=~/^sub\s+(\w+)/gm;say "
+$t{$_} $_" for sort {$t{$b} <=> $t{$a}} keys %t;' $FILE
[download]

This uses precisely* the same logic as your shell loop above (and therefore has the same algorithmic flaws). Nonetheless, advantages of the Perl approach include:

• No multiple forks to grep (one for each sub found)
• Works on multiple input files at once just by appending more to the argument list
• Works on systems without sh, sed, grep, et al.
• Looks like line noise so your cow-orkers will be amazed you can read it let alone write it
• No UUOCA ;-)

Enjoy.

*Not precisely: there's one trivial fix to avoid lines which start with eg. "submarine".