If you're constructing SQL statements, you avoid putting quotation marks around a string that allready been formatted with DBI's quote method.

This is analogous as to how shell arguments constructed using the quotemeta function should be treated, ie: 

$ perl -e '$qs=quotemeta("|"); system( qq{echo $qs} );'

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In reply to Re^3: quoting for system() and friends by snoopy
in thread quoting for system() and friends by pileofrogs

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