$ perl -e '$a="A";$b="B";print "Matches!\n" if "$a$b" =~ /A/'
Matches!

$ perl -e '$a="A";$b="B";print "Matches!\n" if "$a$b" =~ /C/'

$ perl -e '$a="A";$b="B";print "Matches!\n" if $a.$b =~ /A/'
Matches!

$ perl -e '$a="A";$b="B";print "Matches!\n" if $a.$b =~ /C/'
Matches!    # WHY???


$ perl -e '$a="A";$b="B";print "Matches!\n" if "$a$b" eq "AB"'
Matches!

$ perl -e '$a="A";$b="B";print "Matches!\n" if "$a$b" eq "C"'

$ perl -e '$a="A";$b="B";print "Matches!\n" if $a.$b eq "AB"'
Matches!

$ perl -e '$a="A";$b="B";print "Matches!\n" if $a.$b eq "C"'