All the previous answers are far superior to what I'm about to say, but in the spirit of TMTOWTDI:

#!/usr/bin/perl -p
s/,/\t/g;
END{warn "Changed , to tab on $. lines\n"}
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or, for further obfuscation,

#!/usr/bin/perl -naF,
print join "\t",@F;
END{warn "Changed , to tab on $. lines\n"}
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perl looks at any switches in the shebang line, which can ease translating one-liners into scripts. It can drive any future maintainers crazy though.