Another trick to reduce processing time is to compose all the  @strings_to_be_matched strings into a single regex.

c:\@Work\Perl\monks>perl -wMstrict -le
"my @strings_to_be_matched = qw(foo bar wibble wobble fee_fie foe fum)
+;
 ;;
 my ($ur) =
   map  qr{ $_ }xms,
   join q{ | },
   map  quotemeta,
   reverse sort
   @strings_to_be_matched
   ;
 print qq{\$ur: $ur};
 ;;
 my $reg3 = qr/extern.+\b$ur\b\s*/i;
 print qq{\$reg3: $reg3};
"
$ur: (?^msx: wobble | wibble | fum | foo | foe | fee_fie | bar )
$reg3: (?^i:extern.+\b(?^msx: wobble | wibble | fum | foo | foe | fee_
+fie | bar )\b\s*)
[download]

I'm making a couple of assumptions:

• the  @strings_to_be_matched strings are all C/C++ or similar keywords or identifiers and so consist entirely in  \w characters;
• the  $ur pattern is always bounded by  \b assertions whenever it is interpolated.

If these assumptions are true, then a couple of steps in creating the  $ur regex are redundant, but will do no harm.

So your final code might look something like this (untested):

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) { ... }

    }
[download]

(But please see Discipulus's remarks above about using a while-loop rather than a for-loop for processing file contents line-by-line.)

Another thing that may affect speed is that you have all your regexes  $reg1 $reg2 $reg3 $reg4 $reg5 modified as  /i (case insensitive). Case insensitivity slows down regex execution. Some of your regexes have only assertions, characters or character classes  \S $ ; = to which case insensitivity does not apply. As noted above, the  @strings_to_be_matched strings seem to be C/C++ or suchlike keywords or identifiers; is case insensitivity ever appropriate here? I would seriously reconsider the use of case-insensitivity.

Last but not least: As a beginner, it's important always to usewarnings; and usestrict; and avoid global variables.

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