Have you considered using Proc::ProcessTable instead of doing it yourself?

Update: but for a clue as to what you are doing wrong, try capturing _all_ of the matches in an array and printing that.

foreach ( @i ) {
  my @proc = /^
            (\w+)            # capture username
            \s+
            (\d+)            # capture PID
            \s+\d+\s+\d+\s+
            (?:              # cluster (not capturing)
              (\d{4})              # capture %Y
              |                    # or
              (\d{2}:\d{2})        # capture %H:%M
              |                    # or
              (\d{2}:\d{2}:\d{2})  # capture %H:%M:%S
              |                    # or
              (\w{3}\d{2})         # capture %b%d
              |                    # or
              (\w{3}\s+\d{2})      # capture %b %d
            )
            \s+\S+\s+\S+\s+  # skip 2 columns after the 5th column
            (.*)             # capture the command
    $/gx;

    print join ' / ', @proc;
    print "\n";
}
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