Okay, how about this for a solution?  It doesn't do it all in a single regex (hey, go easy on me; I'm only at monk level), but it does do what I think it is you're asking for ...

#!/usr/bin/perl -w

use strict;
use warnings;


# Prototypes
sub findMatches($$);


# Input data
my @keywordList = (
    'john',
    'john.smith',
    'john.smith@mail.com'
);


# Main program
my $searchText = "john's username is john.smith and his email address 
+is john.smith\@mail.com";

my $pmatches = findMatches($searchText, \@keywordList);
map { print "$_\n"; } @$pmatches;


# Subroutines

#
#   Inputs:  $1 ... the text string to match against
#            $2 ... a pointer to the list of valid matching substrings
#
#  Outputs:  $1 ... a pointer to a list of all matches
#
sub findMatches($$) {
    my ($text, $plist) = @_;
    my @matches;
    foreach my $pattern (@$plist) {
        while ($text =~ /($pattern)/g) {
            my $result = $1;            # Got a match in $1
            push @matches, $result;        # Save it to our master res
+ults list

            # Now trim off the first character (otherwise we'll be mat
+ching
            # against the same substring (think 'deep recursion'), and
+ call
            # this subroutine again for recursively generated sub-matc
+hes.
            # Whatever we get (if anything) is added to the list.
            #
            $result =~ s/^.//;
            my $psubstr = findMatches($result, $plist);
            push @matches, @$psubstr;
        }
    }
    return \@matches;
}
[download]

By the way, you're a java programmer, right?  I'm not being perjorative or anything (well, maybe a little :-), but I had never heard of a "reluctant qualifier", and when I googled for it, I got mostly Java matches.  That plus the lowerCaseMixedWithUpperCase variable names kinda gave you away ... ;->