#!/usr/bin/perl 
use strict; 
use warnings; 
 
print "Starting script\n\n"; 
 
my $file_name = "test.txt"; 
 
my $pos  = 0; 
 
open (FILE, $file_name) or die ("File could not be found!!"); 
 
while (<FILE>) { 
		   	    
		   if (/pattern/) { 
 
			$pos = tell(FILE); 
                        $line = $.; 
# Get length of current line in bytes (characters) 
                        $length_of_line = #Need help here to calculate this 
#Calculate offset 
                         $offset = $length_of_line + $pos; 
#Now go to next line	(offset from current location)		 
			seek(FILE, $offset, 1)     or die "Couldn't seek to $offset: $!\n"; 
			 
#Now perform the substitution 
			 
			} else {next}; 
} 
 
print "End script\n"; 
 
close(FILE);

#!/usr/bin/perl 
 
use strict; 
use warnings; 
 
print "Starting script\n\n"; 
 
my $file_name = "test.txt"; 
open my $fh, '<', $file_name or die "could not open '$file_name' $!"; 
 
while ( my $line = <$fh> ) { 
    if ( $line =~ /your pattern/ ) { 
        print $line; 
        my $next_line = <$fh>; 
        $next_line =~ s/change this/to something/; 
        print $next_line; 
    } 
    else { 
        print $line; 
    } 
} 
close $fh;