If you wanted to shorten up the regex, you could do something like this:

my $str = "R4:abcxyz45:LNX";
my ($servername) = ($str =~ /:(.+):/);
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However, I should point out that this has a greater potential to create problems than the regexes offered by kennethk. In other words, the above has shorter keystrokes, but kennethk's solutions are safer.