Ok, first get the line numbers, 

my %line;
{
    open my $fhA, '<', 'A' or die $!;
    while (<$fhA>) {
        $line{$.} = undef if /$X/;
    }
}

[download]
The $. variable counts line numbers.

Now open up file B and do the substitution if $line{$.} exists. Keep all the lines of the revised file and write them back when done. 

use Fcntl qw/:flock/;
{
    my @file;
    open my $fhB, '+<', 'B' or die $!;
    flock $fhB, LOCK_EX or die $!;
    while (<$fhB>) {
        s/$Y/$Z/ if exists $line{$.};
        push @file, $_;
    }

    truncate $fhB, 0 or die $!; # set up to overwrite file
    print $fhB @file;
}

[download]
That does it.

I do wonder if your requirement represents a good design. It seems to suppose the the two files will always be in synch. It could be trouble if whatever creates them doesn't ensure that at every moment.

Added: As an alternative, use Tie::File for file B. 

use Tie::File;
tie my @file, 'Tie::File', 'B' or die $!;

{
    open my $fhA, '<', 'A' or die $!;
    while (<$fhA>) {
        $file[$. - 1] =~ s/$Y/$Z/ if /$X/;
    }
}

[download]
That has the advantages of brevity and low memory use.

After Compline,
Zaxo

In reply to Re: This has me stumped, pattern matching, 2 files by Zaxo
in thread This has me stumped, pattern matching, 2 files by tsk1979

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.