The postfix form of if is a statement modifier, not a block modifier. And unadorned curlies form a block.

You could do either:

print( "one\n"), print( "two\n" ) if (1);
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Or:

do{ print "one\n"; print "two\n"; } if (1);
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Though there is little advantage of the latter over:

if( 1 ){ print "one\n"; print "two\n"; }
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But most people will generally dislike it if you do either.

As an aside, your example is not a good one, as two print statements can always be combined:

print "one\ntwo\n" if (1);
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Personally, I find that there are cases for which compounding statements with a postfix if makes sense. Eg:

while( my $input = <> ) {
    warn( "Bad input" ), next unless $input =~ m[...];
    ...
}
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