use strict;
use warnings;

my $in =
'123,-->456, 
456,-->234, 
234,-->789, 
789,-->123,
';

print "$1,-->$2\n"
    while $in =~ / (\d{3}) ,--> \d{3}, \s+ (?= \d{3} ,--> (\d{3}) ) /gsx;
#                                          | lookahead assertion  |