my $function = 'b+c(t)+t^234.57*t^n*f(t)+Z';
$function =~ s/n/\$nvar/g;
print "New function: $function\n";

##</code><code>##

New function: b+c(t)+t^234.57*t^$nvar*f(t)+Z

##</code><code>##

# Under Construction