Now I don't get any error but the clipboard is still not working.

What does that mean "not working"?

What happens when you run the test t/zz-GtkTextBuffer.t (does it pass, is the clipboard section skipped)?

gtk-text-buffer-set-text shows text/length just like gtk-clipboard-set-text but zz-GtkTextBuffer.t shows set_text without a length argument, so

What happens if you use  set_text($col1content);?

I click the button 'Copy URL to clipboard' and then I try to paste copied text in text editor--nothing happen.

This is output of test:

1..44
ok 1 - An object of class 'Gtk3::TextBuffer' isa 'Gtk3::TextBuffer'
ok 2
ok 3 - An object of class 'Gtk3::TextBuffer' isa 'Gtk3::TextBuffer'
ok 4 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 5 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 6
ok 7
ok 8
ok 9
ok 10
ok 11
ok 12
ok 13 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 14 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 15 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 16 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 17 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 18 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 19
ok 20
ok 21
ok 22 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 23 - An object of class 'Gtk3::TextChildAnchor' isa 'Gtk3::TextChil
+dAnchor'
ok 24 - An object of class 'Gtk3::TextMark' isa 'Gtk3::TextMark'
ok 25
ok 26 - An object of class 'Gtk3::TextIter' isa 'Gtk3::TextIter'
ok 27 - An object of class 'Gtk3::TextMark' isa 'Gtk3::TextMark'
ok 28 - An object of class 'Gtk3::TextMark' isa 'Gtk3::TextMark'
ok 29
ok 30
ok 31 - An object of class 'Gtk3::TextTag' isa 'Gtk3::TextTag'
ok 32
ok 33
ok 34 - An object of class 'Gtk3::TextTag' isa 'Gtk3::TextTag'
ok 35 - An object of class 'Gtk3::TextTag' isa 'Gtk3::TextTag'
ok 36
ok 37 - An object of class 'Gtk3::TargetList' isa 'Gtk3::TargetList'
ok 38 - An object of class 'Gtk3::TargetList' isa 'Gtk3::TargetList'
ok 39 - An object of class 'Gtk3::TargetList' isa 'Gtk3::TargetList'
ok 40 - An object of class 'Gtk3::TargetList' isa 'Gtk3::TargetList'
ok 41
ok 42
ok 43
ok 44
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If I replace line $clipboard->set_text($col1content, length $col1content); with $clipboard->set_text($col1content); I get an error message:

$ perl clipboard.pl
$tpath --> ARRAY(0x9e5e740)
$tp --> Gtk3::TreePath=SCALAR(0x9e5e730)
$col0content --> Google
$col1content --> http://www.google.cz/
$clipboard --> Gtk3::Clipboard=HASH(0xa517230)
*** unhandled exception in callback:
***   Gtk3::Clipboard::set_text: passed too few parameters (expected 3
+, got 2) at clipboard.pl line 91.
***  ignoring at /usr/share/perl5/vendor_perl/Gtk3.pm line 321.
$
[download]

Hmm

What happens if you don't specify an atom? Or use "PRIMARY" as the atom?