use strict;
use Data::Dumper;

sub foo {
  my %opt = ( text  => 'Default Text'
            , style => 'Default Style'
            , bar   => 'belch'
            );
  %opt = ( %opt, @_ );

  print Dumper \%opt;

}

foo();               # Options stay as specified above.
foo( bar => 'yuk' ); # $opt{text} and $opt{style} stay the same, 
                     # $opt{bar} becomes 'yuk' instead of 'belch'.