• <${bcsdir}*> is a "File Glob" (like with ls $bcsdir* in a unix shell)
• $bcsdir = (...)[0] is a "List Slice" which returns the first element of the inner result list ...
• (... or an empty list (resp undef in scalar context) if the result is empty)

HTH! :)

update

here a little demo in the debugger

> perl -de0
  ...
  DB<15> $path = 'D:/tmp'

  DB<16> x <$path/x*>
0  'D:/tmp/xref.txt'
1  'D:/tmp/xref_trace'
  DB<17> x (<$path/x*>)[0]
0  'D:/tmp/xref.txt'
  DB<18>
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