#!usr/bin/perl -w
use warnings;

sub modify_list
{
   my $listRef = shift;
   @$listRef = grep{!/^b/}@$listRef;
   #note there is no "return" value here
   #the list has been modified!!!!
}

my @list = ("a1", "b23", "c45", "d1", "b43", "b65");
print join (" ",@list),"\n"; 
#prints: a1 b23 c45 d1 b43 b65

modify_list(\@list);
print join (" ",@list),"\n"; 
#prints a1 c45 d1