my $do_operation; # Defined elsewhere, possibly from user input
my $by_ammount; # Also from user
my @employees; # Also defined elsewhere
my $op = 
    $do_operation eq 'add'   ? '+' :
    $do_operation eq 'mult'  ? '*' : 
    # And so on
;
eval "\$_->salary $op= \$by_amount" for @employees;