It sounds like you're just wanting to remove any periods that follow just one word character. I'm sure there might be a rare exception to this, but the following would accomplish that basic feat:

use strict;
use warnings;

my $str = "a history of u.s. coast guard aviation.  M.C. Esher";

$str =~ s/(?<=\W\w)\.//g;

print $str, "\n";
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I'd also be tempted to force capitalization on those abbreviations, but that might not be what you're after:

# Remove periods and capitalize, so we have US instead of us.
$str =~ s/(?<=\W)(\w)\./\U$1/g;
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- Miller