use strict;

my $a = 1;

change($a);
print "a is now: $a\n";

not_change($a);
print "a is now: $a\n";

sub change{
    ++$_[0];
}

sub not_change{
    my $b = shift;
    ++$b;
}
__OUTPUT__
a is now: 2
a is now: 2