In the case of perl-base, dpkg is a Pre-Depends, not a Depends. What's Pre-Depends?
This field is like Depends, except that it also forces dpkg to complete installation of the packages named before even starting the installation of the package which declares the pre-dependency[...].
Pre-Depends should be used sparingly, preferably only by ...
Indeed, the Nextcloud snap bundles everything it needs, including PHP v7.2. v7.2 is still supported and receiving regular updates, and the snap maintainers (disclaimer: that's me) don't typically introduce changes without a reason. However, v7.2 will reach the end of active support in a month, which is probably why the app you mention notes that it will no ...
The first step to solve your issue is to revert the effects of your commands.
Let's start with your first command.
echo "deb http://www.rabbitmq.com/debian/ testing main" | sudo tee /etc/apt/sources.list.d/rabbitmq.list > /dev/null
This is creating a the file /etc/apt/sources.list.d/rabbitmq.list. So you want to delete this file. To delete the file ...
DR14 T.meter Official Web Page
DR14 T.meter Official DEB Package (current version 1.0.15)
DR14 T.meter source on GitHub
Install on Ubuntu 19.10
# Dependencies from Ubuntu Repositories
sudo apt install lame flac vorbis-tools faad dh-python python-scipy python-numpy
# DR14 T.medter can draw spectrograms and other graphs using python-...