#!/usr/bin/perl

use strict;
use warnings;

my $file = shift or die;      # read csv file...
my $hash;

open my $fh, "<", $file;
my $result = <$fh>;           # grab header...
while ( my $line = <$fh> ) {  # then read rest of file.
  push @{$hash->{ [ split /,/, $line ]->[1] }}, $line;
}

$result .= join '', @{$hash->{$_}}
  for sort keys $hash;

print $result;

__output__
CAT_HEADER,SUPPLIER_CODE,CUSTOMER_CODE
CAT_LINE,0001P,ABC12345,20190924,,1,Z,3.36
CAT_LINE,0001P,ABC23456,20190924,,1,Z,2.24
CAT_LINE,0001P,ABC34567,20190924,,1,Z,2.24
CAT_LINE,0002P,ABC12345,20190924,,1,Z,3.36
CAT_LINE,0002P,ABC23456,20190924,,1,Z,2.24
CAT_LINE,0002P,ABC34567,20190924,,1,Z,2.24