$values = "'abc', 'dec'f', ''ghc''" ;

$values =~ s/
  (?<!\A)     # no beginning-of-string in front
  (?<!,\s)    # no comma and space in front
  \'          # quote
  (?!,\s|\z)  # no command and space or end-of-string ahead
 /
  \\\'        # replace with \'
 /xg;