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    $groups{$_}{left}   = $rxn[0] for @left_els;
    $groups{$_}{right}  = $rxn[0] for @right_els;

use strict;
use warnings;
...
R00081MM#R00081#C00007MM + 4 C00126MM + 8 C00080MM <=> 4 C00125MM + 2 
+C00001MM + 4 C00080Cyto
R00086MM#R00086#C00008MM + C00009MM + 7 C00080Cyto <=> C00002MM + C000
+01MM + 7 C00080MM
R00094MM#R00094#2 C00051MM + C00003MM <=> C00127MM + C00004MM + C00080
+MM