Assuming the data is sorted on the first column, and read from <>, I'd write it as:

#!/usr/bin/perl

use strict;
use warnings;

my $file = "";
my $fh;

while (<>) {
    my ($node, $mess) = split /,/ => $_, 2;
    unless ($node eq $file) {
        close $fh or die "close $file: $!" if $fh;
        $file = $node;
        open $fh => '>', $file or die "open $file: $!";
    }
    print $fh $mess;
}

__END__
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Or from the command line:

perl -apF, -e'if($F ne$F[0]){$F=$F[0];open F,">$F"or die;select F}s/[^
+,]+//'
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Abigail