As another option, here's a subroutine to which you send a string and a list of lengths. Matches are returned as an array of arrays (AoA), and the array's empty if nothing's matched:

use strict;
use warnings;

my $str     = 'xxxx yy zzzzz xxxx qqq xxxx vv zzzzz wwww ppp';
my @lengths = qw/2 4 3/;

my @seqs = getSequences( $str, @lengths );

print "@$_\n" for @seqs;

sub getSequences {
    my ( $string, @lengths )   = @_;
    my ( $i,      @sequences ) = 0;

    my $re = join '\b.+?\b', map { $i++; "(?<C$i>[a-z]{$_})" } @length
+s;

    push @sequences, [ map $+{"C$_"}, 1 .. @lengths ]
      while $string =~ /\b$re\b/ig;

    return @sequences;
}
[download]

Output:

yy xxxx qqq
vv wwww ppp
[download]

You mentioned only letters, so [a-z] was used in the regex. However, you may use \\w instead, if that works better for you. Of course, sending the subroutine different lists of lengths produces different results, as the regex is dynamically built.