my @strings_to_be_matched = ...;

my ($ur) =
    map  qr{ $_ }xms,
    join q{ | },
    map  quotemeta,
    reverse sort
    @strings_to_be_matched
    ;
    
my $reg1 = qr/=/i;
my $reg2 = qr/\S+=\S+/i;            
my $reg3 = qr/extern.+\b$ur\b\s*/i;
my $reg4 = qr/;$/i;                       
my $reg5 = qr/.+\b$ur\b\s*/i;

foreach my $ln (@contents_of_file) {

    if ($ln =~ $reg3 and $ln =~ $reg4) { ... }

    if ($ln =~ $reg5 and $ln=~ $reg4 and ($ln !~ $reg1 or $ln =~ $reg2)) { ... }

    if ($ln =~ $reg3 and $ln !~ $reg4) { ... }

    if ($ln =~ $reg5 and $ln !~ $reg4 and $ln !~ $reg1) { ... }

    }