# Using s{}{} form of substitute.
# Substitute supports using several different separator formats
# which helps one avoid having to escape things (like '/') within the pattern.
# The 'x' option which means ignore whitespace so that comments can be easily inserted.
# The 'g' option is global obviously.
# The 'e' option says that the replacement part of the pattern is a perl expression.

$var =~
    s{
        ("[^"]+")       # Matches two quotes and content between them.
                        # Capture the match for use in the replacement.
                        #
                        # Disection of pattern:
                        # ("[^"]+")  = full pattern
                        # (       )  = capture everything between parentheses.
                        #  "     "   = quotes at start and end of pattern.
                        #   [^"]+    = one or more non-quote characters
    }
    {                       # The replacement part is a perl expression.
                            # Original: ($tmp = $1) =~ s/\t/ /g; 
                            # is same as next 2 lines:
        $tmp = $1;          # Make a copy of the captured match.
        $tmp =~ s/\t/ /g;   # Replace tabs with spaces throughout the match.
        $tmp;               # Use resultant value for replacement.
    }xge;
    # x = ignore white space and comments
    # g = global
    # e = expression