It's easiest to understand if you realize that:

$var !~ s/H/W/;
[download]

is equivalent to:

(not ($var =~ s/H/W/))
[download]

That is, the only difference between !~ and =~ is the return value. The side effect of mutating the left hand side when evaluated is the same for both operators.

That is, !~ followed by a s/// expression can screw up the contents of the left hand side just as well as though you had used =~

This also explains the "null context" warnings. Since !~ has the same effect as =~ when you're ignoring the return value (as you are here), perl lets you know that you're probably doing something you didn't intend to do - people evaluate =~ just for the side effects all the time, but using !~ that way is a bit odd. Note that this code doesn't emit the same warning:

#!/usr/bin/perl -w
use strict;
 
my $var = "WooHoo\n";
 
print $var if $var =~ /^Woo/;
 
print $var if $var !~ /^Hoo/;
 
if ($var =~ s/W/H/) {print "=~ succeeded\n";}
else {print "=~ failed\n";}
print $var;

if ($var !~ s/H/W/) {print "!~ succeeded\n";}
else {print "!~ failed\n";}
print $var;
[download]

--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/
[download]