You may prefer this line-by-line approach:

C:\Users\Bill\forums\monks>type tj999.pl
use strict;
use warnings;
open my $INFILE, '<', \<< 'END_INFILE';
123,-->456,
456,-->234,
234,-->789,
789,-->123,
END_INFILE

my $old_line = <$INFILE>;
while (my$new_line = <$INFILE>) {
    my $sample = $old_line.$new_line;
    $sample =~ /^(\d{3}).*(\d{3})\,\s*$/s;
    print "$1,-->$2\n";
    $old_line = $new_line
}

C:\Users\Bill\forums\monks>perl tj999.pl
123,-->234
456,-->789
234,-->123
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