Is the ncurses-dev package (and ncurses5-dev etc) not available in Trisquel?

> I can't find ncurses-dev or ncurses5-dev in synaptic

Libraries often begin with 'lib' and aren't always under the name you'd expect. I find it helpful to search by substrings that should be in the name.

$ apt-cache search ncurses5 dev
lib32ncurses5-dev - developer's libraries for ncurses (32-bit)
libncurses5-dev - developer's libraries for ncurses
libx32ncurses5-dev - developer's libraries for ncurses (x32)
lib64ncurses5-dev - developer's libraries for ncurses (64-bit)

$ sudo apt install libncurses5-dev

> I also can't find perl5.

$ apt-cache search perl5
libconvert-binhex-perl - Perl5 module for extracting data from macintosh BinHex files
libdbd-mysql-perl - Perl5 database interface to the MySQL database
libfinance-streamer-perl - Perl5 module with interface to Datek Streamer
libmagpie-perl - RESTful Web Framework for Perl5
libmath-numbercruncher-perl - Perl5 module with commonly needed Maths and Stats functions
libmime-tools-perl - Perl5 modules for MIME-compliant messages
libnet-xwhois-perl - Whois Client Interface for Perl5
libow-perl - Dallas 1-wire support: Perl5 bindings
libperl5.22 - shared Perl library
libperl5i-perl - pragma to fix as much of Perl 5 as possible
libperl6-junction-perl - module providing Perl6-style Junction operators in Perl5
libsnmp-info-perl - Object Oriented Perl5 Interface to Network devices and MIBs through SNMP
libsnmp-perl - SNMP (Simple Network Management Protocol) Perl5 support
libsyntax-keyword-junction-perl - Perl6 style Junction operators in Perl5
libtext-microtemplate-perl - Micro template engine with Perl5 language

$ sudo apt install libperl5.22