$ cat t.pl
#!/usr/bin/perl
use strict;
use warnings;

local $/ = '?srt=';
while (<DATA>) {
    chomp;
    print "$1\n" if m[-/(\w{5,20})$];
}
__DATA__
...emable-Stuff-10100-PTZ-/1280640AB018292?srt=More2stuff&ha...
...wer-Idaptx-SJ10-/35DE4715844?srt=L12_Defa43Dom..
foo-/a?srt=bar;foo-/abcdefghijklmnopqrstuvwxyz?srt=
foo-/abcde?srt=bar-/abcdef?srt=baz-/abcdefg?srt=

$ perl t.pl
1280640AB018292
35DE4715844
abcde
abcdef
abcdefg