#!/usr/bin/perl
use Tk; 
print $Tk::VERSION,"\n";

# 1-liner
# perl -mTk -e 'print $Tk::VERSION'

# On Windows one needs double quotes.
# perl -mTk -e "print $Tk::VERSION"
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But Ubuntu is one of those distros that have had problems with an unpatched Tk version. Try to install Tk with latest patches