This is probably neither the most graceful nor most efficient way to do it, but all you need to do is store your expression in a variable and eval the variable whenever you want to use it. For example:

#!/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";
}
[download]

output:

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

hope this helps,

davidj