I will try to explain in detail. When I post on PerlMonks, I try to be a bit more verbose than when I'm writing my own code, but I also try to show brief shortcuts...

# don't leave home without these ;)

use warnings;
use strict;
 
# this could be from a file or whatever

my @lines = (
    "myserver.it.ca. XXXX XXX XXX XXXXXXXXXXX",
    "myserver.it.org. XXXX XXX XXX XXXXXXXXXXX",
    "myserver.it.com. XXXX XXX XXX XXXXXXXXXXX",
);

# in the next line, we take every array element of the above @lines,
# and work on them one at a time. $line contains each
# of the array elements, in order

for my $line (@lines){
    
    # the below split() is a bit of trickery. we split the line on 
    # whitespace (\s) (the + is superfluous in this case, it means
    # "one or more" (you had a tab), so we split on ANY whitespace).
    # 
    # because I wrapped the split() within parens (()), that forces
    # it into list context, so I treat it as an array (of sorts),
    # and after the split() executes, I can immediately take
    # the first element of it ([0]), and assign it to $server. 
    #
    # that isn't anything special... it's just avoiding splitting
    # into a named array, then having to take the first element
    # from that and putting it into the variable... two-stage example:
    #
    # my @servers = split /\s/, $line;
    # my $server = $servers[0];

    my $server = (split /\s+/, $line)[0];

    # the following does a substitution... it replaces a '.' 
    # (dot, period) character at the very end of $server with nothing 
    # (ie. it removes it) in your OP, you had s/.$//. In Perl 
    # (and in sed), a '.' represents *any* character (perl doesn't 
    # recognize a '.' as a newline by default, I don't know about sed)
    # I changed it so it looks for a dot explicitly by escaping it 
    # with a backslash (\). After the substitution, we print...

    $server =~ s/\.$//;
    print "$server\n";
}
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