perl -d foo.pl
...
Loading DB routines from perl5db.pl version 1.3
Editor support available.

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

main::(foo.pl:9):	my $limit = 100_000_000;
  DB<1> n
main::(foo.pl:10):	my $zip_file = "zipfile.zip";
  DB<1> p $limit
100000000
  DB<2>