If your three lines are not in the string buffer you are matching against already, you cannot solve this with regular expressions. I would maintain a "skip" count to know how many lines to skip:

my $skip;
while (<>) {
    next
        if $skip--;
    if( /mypattern/ ) {
        $skip= 3;
    } else {
        print "Have line '$_'";
    };
};
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If your three lines are in the string buffer already, a new line is defined by the newline character(s) appearing. So you would match three newline charachters with non-newline characters in between them to skip three lines forward.

Without seeing code, it's hard to advise better.

perl -pe 's/.*pattern\n$/<>for 1..3;$_/e'
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My understanding is that you want to print the two lines after the match? Then:

use strict;
use warnings;
open my $fh, "<", "somefile.txt";
while(my $line = <$fh>){
    if($line =~ /pattern/){
        print "Matched: $line";
        print $line if defined( $line = <$fh>);
        print $line if defined( $line = <$fh>);
    }
}
close($fh);
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read perlintro

#!/usr/bin/perl
use strict; use warnings;

my $file;
my $match = "that";
my $extra = "Hello, I snook in here\nme too!";
{
   local $/;
   $file = <DATA>;
   $file =~ s/$match\n([^\n]*)\n([^\n]*)/$match\n$1\n$2\n$extra/;
}
print $file;

__DATA__
this
that
leave me
leave me
move me
and me
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this
that
leave me
leave me
Hello, I snook in here
me too!
move me
and me
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Cheers,
R.

I opened the file in string context and was able to find the pattern successfully. But I could not substitute the matched pattern with required pattern

$extra = "This is i will place after match\n.an this too";
open (my $fh, '+<', $file);
$data = <$fh>;
$data = s/(.*entry)\n([^\n]*)\n/$1\n$extra/
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The $1 is found successfully. But could not do the substitution in the file.

You have now substituted within the $data variable. Next you need to write the contents of $data back to the file. Try adding something like this after the substitution

# untested
seek  $fh, 0, 0; # rewind to the start of the file
print $fh $data; # Write the entire file
close $fh;
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Of course this may not be very efficient if you have a large file to alter, but then inserting into the middle of a very big file never is. If you are frequently updating random records in a file you may need to start thinking about using a database.

Cheers,
R.

Pereant, qui ante nos nostra dixerunt!

Yes there is one line where we can do match out of the three lines and that line is the first line in three lines.

The following code uses an array as a "circular list" to delay the printing of the EXTRA. Is this what you need?

use strict;
use warnings;
my $match = qr /Match/;
my $extra = qq(EXTRA\nEXTRA\n);
my @extras = (q()) x 3;
my $in  = 3;
my $out = 0;
while (my $line = <DATA>) {
    $out = ($out+1) % 4;
    print $line, $extras[$out];
    $in  = ($in +1) % 4;
    $extras[$in] = ($line =~ /$match/)? $extra : q();
}
__DATA__
a
b
Match1
Match2
c
d
e
f
g
Match3
h
i
j
k
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Bill