It's generally a good idea to put a leading space before the actual name of the file/command, not before any control characters that determine how the filehandle should be opened. Consider:

# this is pseudo-code
$filename = '>test' or '+test'
open(HANDLE, ">$filename");   # >>test or >+test (ruh-roh!)

# likewise
$filename = 'ls|' or '>test;
open(HANDLE, "$filename");    # ls| (pipe) or clobbers 'test'

# generally safe (in that behavior is not unexpected):
open(PIPE, " $command |")
open(PIPE, "| $command\0");   # safe version of "|ls"
open(FILE, "> $filename\0");  #                 ">file"
open(FILE, " $filename\0");   #                 "file"
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In the last case, the presence of a space doesn't affect what is open, but since the first character is a space, any leading "mode" characters in $filename will be ignored, much like what was happening with your code.

Also see sysopen and perlipc (e.g., using fork/exec), which (in my opinion) are significantly more readable/safer than using weird manglings of open. Note that issues of shell meta-characters and other security considerations are out of the scope of this message. Each of the "PIPE" examples above are still incredibly insecure if untrusted input is allowed to use any shell metacharacters in the string. See perlsec and use Taint-checking.