#!/usr/bin/perl
#
my $expr = '$a + $b';
 
my $a = 4;
my $b = 3;
print eval $expr, "\n";
 
$a = 6;
print eval $expr, "\n";
 
if( (eval $expr) == 9 ) {
    print "eval \$expr = 9\n";
}

##</code><code>##

247.~/perl/tmp > a.pl
7
9
eval $expr = 9