package TT;

use Exporter;
@ISA = qw/Exporter/;

@EXPORT = qw/s1/;

sub s1 {
  print "s1";
  s2();
}

sub s2 {
  print "s2";
}


1;


perl -wle 'use TT; sub s2 { print "NO S2!"; } s1();' 

output:
s1
s2

I want: 
s1
NO S2! ?