You can use sysread to read any available bytes from the stream without worrying about whether there is a newline in there or not. Put whatever you read into a buffer in case it was only part of a transaction...

#!/usr/bin/perl -w
use strict; use warnings;
use IO::Socket;
use IO::Select;

my $listener = IO::Socket::INET->new(
    LocalPort   => 42424,
    Listen      => 1,
) or die "failed to create socket: $!";
my $select = IO::Select->new( $listener );

my %buffers = ();
while ( my @ready = $select->can_read ) {
    foreach my $fh ( @ready ) {
        if ( $fh == $listener ) {
            my $new = $listener->accept;
            $buffers{ $new } = '';
            $select->add( $new );
        } else {
            $fh->sysread( $buffers{ $fh }, 1024, length( $buffers{ $fh
+ } ) );
            while ( $buffers{ $fh } =~ s/STX(.*?)ETX// ) {
                process( $1 );
            }
        }
    }
}
sub process {
    print "GOT: $_[0]\n";
}
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