In the spirit of laziness as it pertains to maintaining/adding-things-to code, I would rewrite the first example to automatically generate the words to be replaced from the keys in the hash (and, I was just a little bored, so I did this).

my %ch = ('green' => 'lousy',
         'blue'  => 'cool',
         'pink'  => 'mini',) ;
my $str = 'I have a green hat, blue shirt, plus a pink jacket';
print $str . "\n" ;
my $keyList = '('. (join '|', keys %ch) . ')';
my $regex = qr/$keyList/;
$str =~ s/$regex/$ch{$1}/g;
print $str ;
[download]

That way, if we wanted to change all instances of 'hat' to 'fedora', we just add it to the hash and go about our business.

my %ch = ('green' => 'lousy',
         'blue'  => 'cool',
         'pink'  => 'mini',
         'hat'   => 'fedora') ;
my $str = 'I have a green hat, blue shirt, plus a pink jacket';
print $str . "\n" ;
my $keyList = '('. (join '|', keys %ch) . ')';
my $regex = qr/$keyList/;
$str =~ s/$regex/$ch{$1}/g;
print $str ;

__END__

I have a green hat, blue shirt, plus a pink jacket
I have a lousy fedora, cool shirt, plus a mini jacket
[download]

TIMTOWTDI,
enoch