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

16:28 >perl 1814_SoPW.pl
123,-->234
...
234,-->123

16:28 >