You can use getppid resp. Win32::Getppid to find the PID of the parent process. This is not necessarily a shell, because Perl and other programs can also launch Perl directly without an intervening shell.

$ perl -E 'my $parent = getppid();
           open my $fh, "<", "/proc/$parent/comm";
           print <$fh>'
bash
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... with the caveat that orphaned processes get adopted by init (process 1). Might want to test for that.

Also, there's a potential for race in there.

#!perl
use strict;
use warnings;
use if ($^O eq 'MSWin32'), 'Win32::Getppid';

my $parent;
my $invoker;


BEGIN{
  if ($^O eq 'MSWin32') {
      $parent = getppid;
      open my $cmd,"tasklist  /nh /FI \"PID eq $parent\" |" or die;
      while (<$cmd>){
            chomp;
            next if /^$/;# yes it outputs a leading empty line..
            $invoker = $1 if /^([\w\d\.]+)\W/;
      }
  }
  else {
       $parent = getppid;
       open my $cmd,"ps -p $parent|" or die;
       while (<$cmd>){
            chomp;
            next if /^[\sA-Z]+$/;# get rid of heaader: freebsd does no
+t support no header output
            $invoker = $1 if /\s([\w\d\.]+)$/;
       }
  }
}


print "Perl (PID $$) was invoked from PID $parent ($invoker)\n";
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cmd.exe>perl who_called_Perl.pl
Perl (PID 17400) was invoked from PID 10576 (cmd.exe)

powershell> perl.exe who_called_Perl.pl
Perl (PID 18212) was invoked from PID 11272 (powershell.exe)

bash$ perl who_called_perl.pl 
Perl (PID 73288) was invoked from PID 31994 (bash)
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