# open the input file
open(IN,"file") or die;
#
# open a temporary output file ( you could also use the
# -i "inplace edit feature", see perldoc perlrun )
#
open(OUT,">file.$$") or die;
#
# print the contents of IN to OUT,
# adding your additional line if we
# see $dom as the first chars of the line
# \Q and \E to quotemeta the domain since
# it probably contains metachars like "."
#
while (<IN>) {
    print OUT;
    if (/^\Q$dom\E\s/) {
          print OUT "$user\@$dom\t$user\n";
    }
}
# close both files and overwrite
# IN with OUT via rename().
close IN;
close OUT;
rename("file.$$","file") or die;
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Update: Regarding the reply...other than the fact that it's opening a file called "file", the above is doing what you are asking. I think perhaps you are focused on the word seek. The seek() operator doesn't look for strings, it's given a byte offset and moves the file pointer.

Also, you seem fixated on editing the file in place. This isn't really possible in your situation. Perl can make it appear that way with it's "-i" feature. See perlrun.

my original code (before i changed the layout of the file) looked like this:

my $datafile = '/etc/postfix/virtual';
my $req_addr = "$username\@$domain";
my ( $name, $atdomain ) = $req_addr =~ /^([\w.-]+)(@[\w.-]+)$/ ;
open FH, $datafile or die;
my @virtual = <FH> ;
close FH ;
open FH, ">$datafile" or die;
my $wrote_it = 0 ;
for ( @virtual )
{
  print FH $_ ;
  if ( $_ =~ /$atdomain/ && !$wrote_it )
  {
          print FH "$req_addr\t$username\n" ;
    $wrote_it++ ;
  }
}
close FH ;
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Which would edit the /etc/postfix/virtual file if it looked like this:

aaaa.com        VIRTUALDOMAIN
@aaaa.com        postmaster


bbbb.com     VIRTUALDOMAIN
@bbbb.com    postmaster

cccc.com    VIRTUALDOMAIN
@cccc.com   postmaster
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I've seen that exact same code before (apart from the two lines that now have $atdomain instead of $domain), with the same question.

I mumbled something about Tie::File. That might still be a nice solution.

— Arien

judging by what you have there, i don't think that i explained it correctly.
I need the script to do the following:

1. open /etc/postfix/virtual (which is arranged as above)
2. seek to the line that begins with the same value as $dom
3. do a return at the end of that line and print $user\@$dom\t$user

Hope this is clearer. (i'm crap at explaining).
Thanks

so if $user is bob and $dom is aaaa.com, the file will look like below once it has been wrote to:

aaaa.com        VIRTUALDOMAIN
bob@aaaa.com    bob

bbbb.com     VIRTUALDOMAIN


cccc.com    VIRTUALDOMAIN


dddd.net      VIRTUALDOMAIN
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Thanks

my $infilename = "/etc/postfix/virtual";
open my $infile, "<", $infilename or die "Can't open $infilename to re
+ad: $!\n";
open my $outfile, ">", "$infilename.new" or die "Can't open $infilenam
+e.new to write: $!\n";
local $_;
while(<$infile>) {
    print $outfile $_;
    print $outfile "$user\@$dom\t$user\n" if /^\Q$dom\E\b/;
}
close $infile;
close $outfile;

rename("$infilename.new", $infilename) or die "Couldn't replace $infil
+ename with $infilename.new: $!\n";
[download]

Makeshifts last the longest.

@mta = <MTA_FILE>;
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Twiddle your bits and rewrite your file.

If it is your core, waste it not.
If it is another's core, usurp it not.

Slurp, slurp.