Especially it's suggestion to use the debugger.
Roughly:

> perl -dw prog.pl

Loading DB routines from perl5db.pl

Enter h or `h h' for help.

main::(prog.pl:3):    my $firstline;

  DB<1> c 123

main::(prog.pl:123):    %hololoh # has just been loaded w/ first value
+s

  DB<1> x %hololoh

0  HASH(0x3e5e54)
...
[download]

It's much easier when you can actually look at it with some initial values. (It even makes it easier to get your debugging print statements right.)

p