In the first case, you're splitting on a single space ("my @trace = split("\s", ...").

Actually, it's split-ing on the letter 's' because the  \ (backslash) is an escape in a double-quoted string. The escape is unrecognized but passed through, a fact that Perl would have mentioned had warnings been enabled.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $str = 'the rain in spain falls mainly';
 ;;
 my @ra = split \"\s\", $str;
 printf qq{'$_' } for @ra;
 print '';
 ;;
 @ra = split '\s', $str;
 printf qq{'$_' } for @ra;
"
Unrecognized escape \s passed through at -e line 1.
'the rain in ' 'pain fall' ' mainly'
'the' 'rain' 'in' 'spain' 'falls' 'mainly'
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