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my $s1 = 'Perl';
my $s2 = 'Monks';
my $s3 = "$s1$s2";
print "$s3\n";
# $s3 contains 'PerlMonks'

my $s3 = "${s1}${s2}";

{
   undef $";
...
   print "$s4\n";
   # $s4 contains 'abcde'
}