-bareword is the same as "-bareword".

>perl -e "use Devel::Peek; $x="-bareword"; Dump($x); $x=-bareword; Dum
+p($x)"
SV = PV(0x226150) at 0x226000
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x1835404 "-bareword"\0
  CUR = 9
  LEN = 12
SV = PV(0x226150) at 0x226000
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x1835404 "-bareword"\0
  CUR = 9
  LEN = 12
[download]

There's nothing special about -Infinity.

>perl -e "use Devel::Peek; $x="-Infinity"; Dump($x); $x=-Infinity; Dum
+p($x)"
SV = PV(0x226150) at 0x226000
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x1835404 "-Infinity"\0
  CUR = 9
  LEN = 12
SV = PV(0x226150) at 0x226000
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x1835404 "-Infinity"\0
  CUR = 9
  LEN = 12
[download]

A string that doesn't look like a number is the same as 0 in a numerical context (although it will give a warning if warnings enabled).

The index expression of an array is evaluated in a numerical context.

So they both print 5.

>perl -e"@a = (5 .. 10); print qq{$a['Infinity']\n$a[-Infinity]\n};"
5
5
[download]

Update: There appears to be a bug. 'Infinity' and -Infinity don't trigger the warning.