You don't always need alternation, especially if most of the stuff on either side is the same. You can use the ? quantifier to denote some parts as optional.

This example works for the input you showed, but other forms you might find might require it to change a bit.

#!/usr/bin/perl

$text = <<"HERE";
U.S. Army
the Army has been berry-berry good to me
US-Army
US Army
HERE


$text =~ s/(?:U.?S.?[\s-])*Army/US-Army/g;

print $text;
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