use strict;
use warnings;

my $Orig = '7374,726327,"76,237",32324,"21,342,857",23';
my $Mod = $Orig;

1 while $Mod =~ s/((?:^|,)"[^"]*?),([^"]*?"(?:,|$))/$1$2/g;
print "$Orig => $Mod\n";

7374,726327,"76,237",32324,"21,342,857",23 => 7374,726327,"76237",32324,"21342857",23