The regex to match quoted strings is discussed in detail in Jeffrey Friedl's excellent book: Mastering Regular Expressions. The regex to do this (and many more) is also available in the superb Regexp::Common CPAN module.

Modifying ambrus's solution with Friedl's optimized regex to match a quoted string (which also allows for escaped characters) gives: 

s/("[^"\\]*(?:\\.[^"\\]*)*")/($x=$1)=~y: :_:, $x/ge;

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In reply to Re^2: regexp - replace spaces in quoted string by eyepopslikeamosquito
in thread regexp - replace spaces in quoted string by /dev/luser

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