use strict;

my $chr;
$chr = "this and that";
$chr =~ s/
    (?:
        (this|that)(?{print "matched lh alt: $1 "}) # Left hand alternate
        |
        (\w+)(?{print "matched rh alt: $2 "})       # Right hand alternate
    )
    (?{print 'def 1' if defined ($1); print 'def 2' if defined ($2); print "\n"})
    /$1\U$2/xg;
print "$chr\n";

##</code><code>##

matched lh alt: this def 1
matched rh alt: and def 2
matched lh alt: that def 1
this AND that