This appears to contain a bug, revealed when you change the print lines as shown below:

use strict;
use warnings;

my $list = "This is my list";
$list =~ /
    ^(.+)   # The 'rest' (Everything before last word)
    (\w+)   # Last 'word' (string of contiguous word characters)
    \W*$    # Possible non-word characters at end of string
/x;
my $last = $2;
my $the_rest = $1;
print "the_rest='$the_rest'\n";
print "last='$last'\n";
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the_rest='This is my lis'
last='t'
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There are many ways to fix. Here is one way (adding a \b assertion):

$list =~ /
    ^(.+)    # The 'rest' (Everything before last word)
    \b(\w+)  # Last 'word' (string of contiguous word characters)
    \W*$     # Possible non-word characters at end of string
/x;
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Alternative fixes welcome.