c:\@Work\Perl\monks\tel2>perl -wMstrict -le
"my $s = '123';
 $s =~ s{ }{X}xmsg;
 print qq{A: '$s'};
 ;;
 $s = '';
 $s =~ s{ }{X}xmsg;
 print qq{B: '$s'};
 ;;
 $s = '123';
 $s =~ s{ \z }{X}xmsg;
 print qq{C: '$s'};
"
A: 'X1X2X3X'
B: 'X'
C: '123X'

##</code><code>##

my %globize = ('P' => '{P',  ':' => '}:{', '' => '}');
...
$globule =~ s{ (\A P | : | \z) }{$globize{$1}}xmsg;