Here's some data to illustrate what is happening. I think davido's explanation really hits the nail on the head and Anonymous Monk's suggestion YAPE::Regex::Explain will help you isolate what is happening with your experimentation. BTW, including an asterisk in some of the character classes is redundant, other times, you might see all your asterisks disappear from your input---not what you intended, I think.

ladoix% cat 312141.pl
#!usr/bin/perl
use strict;
use warnings;

my $phrase1 = "This is a test, \"using quotes of 'two different' types
+.\"";
$phrase1 =~ s/[\S\W]*//g;
print "P1: [$phrase1]\n";
my $phrase2 = "This is a test, \"using quotes of 'two different' types
+.\"";
$phrase2 =~ s/[\S\w]*//g;
print "P2: [$phrase2]\n";
my $phrase3 = "This is a test, \"using quotes of 'two different' types
+.\"";
$phrase3 =~ s/[\s\W]*//g;
print "P3: [$phrase3]\n";
my $phrase4 = "This is a test, \"using quotes of 'two different' types
+.\"";
$phrase4 =~ s/[\s\w]*//g;
print "P4: [$phrase4]\n";

my $phrase5 = "This is a test, \"using quotes of 'two different' types
+.\"";
$phrase5 =~ s/[^\s\w]*//g;
print "P5: [$phrase5]\n";
my $phrase6 = "This is a test, \"using quotes of 'two different' types
+.\"";
$phrase6 =~ s/[^\S\w]*//g;
print "P6: [$phrase6]\n";
my $phrase7 = "This is a test, \"using quotes of 'two different' types
+.\"";
$phrase7 =~ s/[^\s\W]*//g;
print "P7: [$phrase7]\n";
my $phrase8 = "This is a test, \"using quotes of 'two different' types
+.\"";
$phrase8 =~ s/[^\S\W]*//g;
print "P8: [$phrase8]\n";


ladoix% perl 312141.pl
P1: []
P2: [         ]
P3: [Thisisatestusingquotesoftwodifferenttypes]
P4: [,"''."]
P5: [This is a test using quotes of two different types]
P6: [Thisisatest,"usingquotesof'twodifferent'types."]
P7: [   , "   ' ' ."]
P8: [This is a test, "using quotes of 'two different' types."]
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