my %CHANGE = (
':-)' => 'happy',
':-(' => 'sad',
);

my $text = "I am :-) but sometimes I am :-(.";

for my $out ( keys %CHANGE ){
   my $in = $CHANGE{$out};
   $text=~s/\Q$out\E/$in/sg; # need to use \Q \E for quoting
   # $text=~s/$out/$in/sg; # <- this will go bonkers, it will 
   # treat $out as a regex, not a string
}

print $text;
# output: I am happy but sometimes I am sad.