#!/usr/bin/perl

require "hage.pl";

my $a = 1;
print $a,"\n";
&hage::routine;
print $a,"\n";

# ↓はhage.pl
#!/usr/bin/perl

package hage;

sub routine {$a = 3;}
1;

では、
$ perl hoge.pl
1
1
となる(安全