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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In reply to Re: trim the blank lines...regex by mr_mischief
in thread trim the blank lines...regex by dani_cv

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