You've already been given good advice, including RTFM which really is good advice here. Yet in the spirit of TIMDOWTDI (some of which don't involve regexes)...

while ( <> ) {
    chomp;
    print "$_\n" if length $_;
}
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{
local $/;
my $contents = <>;
$contents =~ tr/\n/\n/s;
print $contents;
}
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If you're playing golf:

$/='';chomp,print"$_\n"while<>
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print/./?$_:''while<>
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print/./?$_:''for<>
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If you're using one-liners:

perl -ne "print if/./"
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perl -0pe "y/\n/\n/s"
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perl -ne "/./&&print"
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