Hopefully this isn't too obfuscated. I've tried to restrain myself here. The trunc_match function creates a regular expression for any given input string, and should handle wierd stuff to by virtue of the qr() operator.

sub trunc_match
{
        my ($what) = @_;
        my @bits;

        for (1..length $what)
        {
                push (@bits,$what);
                chop $what;
        }

        return '('.join ('|', map { quotemeta($_) } @bits).')$';
}

my $rx = trunc_match ("foobar");

$_ = "This sentence ends in foob";

if (/$rx/)
{
        print "Truncated, ends in '$1'\n";
}
[download]

The format of the regex is something like:

(foobar|fooba|foob|foo|fo|f)$

So you get whatever you're looking for in $1, or the returned array if you're brave enough to use /g.

Update:
For some reason, I had confused qr with quotemeta, and so I am updating the code here to be more sensible in that regard. Thanks, once again, Hofmator.