foreach my $ur (@strings_to_be_matched)
{
     $reg1 = qr/\=/i;
     $reg2 = qr/\S+\=\S+/i;            
     $reg3 = qr/extern.+\b$ur\b\s*/i;
     $reg4 = qr/;$/i;                       
     $reg5 = qr/.+\b$ur\b\s*/i;

     foreach my $line (@contents_of_file)
{

     if(($ln =~ $reg3 and $ln =~ $reg4)){
     <some code>
     }

     if(($ln =~ $reg5 and $ln=~ $reg4 and ($ln !~ $reg1 or $ln =~ $reg2))
     {
     <some code>
     }
     

     if(($ln =~ $reg3 and $ln !~ $reg4)){
     <some code>
     }

     if(($ln =~ $reg5 and $ln !~ $reg4 and $ln !~ $reg1))
     {
     <some code>
     }

   }

}


##</code><code>##

if (($ln =~ $reg5 and $ln=~ $reg4 and ($ln !~ $reg1 or $ln =~ $reg2)))
{
<some code here...>
}

if (($ln =~ $reg5 and $ln !~ $reg4 and $ln !~ $reg1)) 
{
<some code here...>
}