Here are my 2 cents. This code would do what you are asking, but wouldn't handle other problems like a " in the quoted string.

#!/usr/bin/perl -w

my $line = '121212, "Simpson, Bart", Springfield<br>';

print "Before: $line\n";

$line =~ s/"(.*?),(.*?)"/$1_$2/;

print "After: $line\n";
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The output is:

Before: 121212, "Simpson, Bart", Springfield<br>
After: 121212, Simpson_ Bart, Springfield<br>
[download]

To explain, adding a ? after *, as in .*? causes minimal matching. It will match the first ", then as few characters as possible, then a comma, then more of the same until another " is found. It all gets replaced with the s///.