#!perl -l
use strict;
use warnings;

sub routine1 {
  print "Who am I?";
}

sub routine2 {
  print "Where am I?";
}

my %all_subs =
  map  { (\&{$main::{$_}} => $_) }
  grep { defined &{$main::{$_}} }
  keys %{main::}
;

my $some_sub = \&routine1;

if ($all_subs{$some_sub}) {
   print "Found $all_subs{$some_sub}"
}
else {
   print "Sub not found";
}