in reply to showing the error ,Can't modify concatenation (.) or string in scalar assignment

On the left hand side of your assignment

my $check_.$usr = $cgi->param("$chck_.$usr");

you do not have an item that something can be assigned to as it is the concatenation of two strings. You need to define a new variable for that purpose, eg

my $check_usr = $cgi->param("$chck_.$usr");