You can easily write a sub so that it uses $_ if it gets no other arguments.

sub foo {
  my $arg = @_ ? shift : $_;
  ...
}
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Or some such like that...

or maybe &foo to satisfy use strict;

Be very careful about using &foo. That lets foo have access to @_, which might get you in trouble. It's generally better to say foo() or maybe &foo().

Update: The other people who responded have a good point about localizing $_ if you're going to be changing it. But I don't think that's what you were asking for.