There's not enough information for me to test your code; however, this condition strikes me as potentially problematic:

index($buf, "/DATA_String/") > 0
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In "index STR,SUBSTR,POSITION", the POSITION is zero-based. Perhaps you want:

index($buf, "/DATA_String/") > -1
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Consider this code:

$ perl -E '
    my @strings = qw{
        no_data_string
        /DATA_String/
        X/DATA_String/
        /some/path/to/DATA_String/
    };

    say "-" x 40;

    for my $string (@strings) {
        say "\$string[$string]";

        for my $pos (qw{0 -1}) {
            say "\$pos[$pos]";

            if (index($string, "/DATA_String/") > $pos) {
                say "index() TRUE";
            }
            else {
                say "index() FALSE";
            }
        }

        say "-" x 40;
    }
'
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Output:

----------------------------------------
$string[no_data_string]
$pos[0]
index() FALSE
$pos[-1]
index() FALSE
----------------------------------------
$string[/DATA_String/]
$pos[0]
index() FALSE
$pos[-1]
index() TRUE
----------------------------------------
$string[X/DATA_String/]
$pos[0]
index() TRUE
$pos[-1]
index() TRUE
----------------------------------------
$string[/some/path/to/DATA_String/]
$pos[0]
index() TRUE
$pos[-1]
index() TRUE
----------------------------------------
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"I am new to socket programming but found an example in O’Reilly’s “Advanced Perl Programming”. After adjusting the code for my network, ..."

"Advanced Perl Programming" is an excellent book, I own a copy myself; however, it was published over a quarter of a century ago. Here's some suggestions for aligning your code with modern practices:

• Check for I/O exceptions — I'd recommend the autodie pragma.
• Use a lexical variable for the filehandle in the smallest scope possible (see open()).
• Avoid "Indirect Object Syntax".

So, your code might become something like this:

...
use strict;
use warnings;
use autodie;

use IO::Socket;

{
    open my $pid_fh, '>', '/var/run/aws.rcvr.pid';
    print $pid_fh "$$\n";
}

my ($sock, $new_sock, $buf );
$sock = IO::Socket::INET::->new(...);
...
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