evil.pl

use warnings;
use strict;
use Class::Method::Modifiers;

sub myfunc1 {
  return join(' ', @_);
}

sub myfunc2 {
  return join(' ', @_) .' | '. myfunc1('a','b') 
}

warn myfunc2(1,2);

*oldmyfunc1 = \&myfunc1;
around 'myfunc2', sub {
  my $sub = shift;
  my $arg_to_forward = shift;
  local *myfunc1;
  *myfunc1 = sub { &oldmyfunc1(@_, $arg_to_forward) };
  return $sub->(@_);
};

warn myfunc2('c',1,2);

__END__

$ perl test.pl
1 2 | a b at test.pl line 13.
1 2 | a b c at test.pl line 24.