#!/opt/perl/bin/perl
use strict;
use warnings;
use charnames ':full';

binmode STDOUT, ":utf8";
my $string = ':)';

print "$string becomes: ";

$string =~ s/:\)/\N{WHITE SMILING FACE}/;

print "$string\n$string becomes: ";

$string =~ s/\N{WHITE SMILING FACE}/\N{BLACK SMILING FACE}/;

print "$string\n$string becomes: ";

$string =~s/\N{BLACK SMILING FACE}/:\)/;

print "$string\n";
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That should cover most uses of \N{CHARNAME}.

charnames and perlre are the best references.