Another related point is that it's important to use the two constructs under discussion, rather than working on @_ directly (unless that's what you want). @_ is passed in by referecne, most of the time, we work on local copies within the sub.

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
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