"Not working" isn't an error report. As advised in answers to some of your previous questions (e.g. No Such File or Directory Error) you could add use strict; use warnings; and start debugging from there to find out what is going wrong and why.

Great suggestions have been given. Here's are some other observations/suggestions:

• Use a three-argument open.
• Use lexical file handles, e.g., my $fh, instead of FILE.
• Your first while reads all lines from the file, so there are none left for the second while
• Use lexical variables throughout.
• Your if(/\b$s\b/) matches against $_ when you've assigned a file line to $line.
• close the file when your done with it.

Give the above, consider the following:

use strict;
use warnings;

my ( @IPS, $IP );

print "Enter name:\n";
my $s = <>;
chomp $s;

open my $fh, '<', 'routers.txt' or die $!;
while ( my $line = <$fh> ) {

    if ( $line =~ /\b$s\b/ ) {
        my ($IP) = $line =~ /(\d+\.\d+\.\d+\.\d+)/;
        push( @IPS, $IP );
    }

}

close $fh;

print "$_\n" for @IPS;
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Hope this helps!

Well, firstly you read and print the entire file and then try to search in the remaining parts of the file (i.e nowhere). If you really want to work this way, you should seek to the beginning of the file.

Also, your $IP variable is declared inly inside the while loop. Declare it earlier (outside the loop) or print something from @IPS instead.

Also, you need another syntax for your matching:

($IP) = $line =~ /(\d+\.\d+\.\d+\.\d+)/;
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I think it would be easier if you used index() because the IP addresses have '.'s in them which mean 'match any character'.

use feature ":5.14";
use warnings FATAL => qw(all);
use strict;
use Data::Dump qw(dump pp);

my $i = "10.0.0.132";  

say for grep {index($_, $i) != -1} split /\n/, << 'END';
a.AS1    10.0.0.128
b.AS1    10.0.0.129
c.AS1    10.0.0.130
d.AS1    10.0.0.131
e.AS1    10.0.0.132
END
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Produces

e.AS1    10.0.0.132