Actually, if you turn on t at the beginning of your debugging, perl -d does tell you who called the function. Here is the screen shot:

C:\Perl\bin>perl -d b.pl

Loading DB routines from perl5db.pl version 1.25
Editor support available.

Enter h or `h h' for help, or `perldoc perldebug' for more help.

main::(b.pl:1): print "Hello\n";
  DB<1> t
Trace = on
  DB<1> s
Hello
main::(b.pl:2):  a(0);
  DB<1> s
main::a(b.pl:8):            $parameter = shift;
  DB<1> s
main::a(b.pl:9):            if( $parameter ) { die };
  DB<1> s
main::(b.pl:3):  print "Middle\n";
  DB<1> s
Middle
main::(b.pl:4):  a(1);
  DB<1> s
main::a(b.pl:8):            $parameter = shift;
  DB<1> s
main::a(b.pl:9):            if( $parameter ) { die };
  DB<1> s
main::a(b.pl:9):            if( $parameter ) { die };
  DB<1> s
Died at b.pl line 9.
        main::a(1) called at b.pl line 4
Debugged program terminated.  Use q to quit or R to restart,
  use O inhibit_exit to avoid stopping after program termination,
  h q, h R or h O to get additional info.
  DB<1>
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