Ah, is your version of Perl too old to support the /r modifier? It wont work without it. I Suppose: 
my $txt = join ' ',@first_sentence;
$txt =~ s/\s*([.()])\s*/$1/g;
print $txt;

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Updated to simplify regex as suggested by hdb

In reply to Re^3: remove spaces after join operation by Loops
in thread remove spaces after join operation by lakssreedhar

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