keys on scalar warnings and lack thereof

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

perl -e '$_[0]={1,1,2,2};keys %$_[0]'     # no
perl -e '$_[0]={1,1,2,2};print ref $_[0]' # wtf
HASH

perl -e 'keys %$_'.   # ok
perl -e 'keys %$_[0]' # no
perl -e 'keys %$;[0]' # ok
perl -e 'keys %$#[0]' # ok
perl -e 'keys %$^[0]' # ok
perl -e 'keys %$&[0]' # ok
perl -e 'keys %$=[0]' # ok
perl -e 'keys %$|[0]' # ok
perl -e 'keys %$,[0]' # ok
perl -e 'keys %$<[0]' # ok         
perl -e 'keys %$>[0]' # ok

perl -MDevel::Peek -e 'Dump(%$_[0])'
SV = NULL(0x0) at 0x100a29eb8
  REFCNT = 2147483641
  FLAGS = (READONLY,PROTECT)

perl -MDevel::Peek -e 'Dump(%$^[0])'
SV = IV(0x12b025cf0) at 0x12b025d00
  REFCNT = 1
  FLAGS = (PADTMP,IOK,pIOK)
  IV = 5016431784

perl -MDevel::Peek -e 'Dump(%$&[0])'  
SV = IV(0x14a0136f0) at 0x14a013700
  REFCNT = 1
  FLAGS = (PADTMP,IOK,pIOK)
  IV = 0

perl -MDevel::Peek -e 'Dump(%$^[0])'  
SV = IV(0x1590136f0) at 0x159013700
  REFCNT = 1
  FLAGS = (PADTMP,IOK,pIOK)
  IV = 5779793576

perl -MDevel::Peek -e 'Dump(%{$^[0]})'
Number found where operator expected (Missing operator before "0"?) at
+ -e line 1, near "$^[0"
syntax error at -e line 1, near "$^[0"

perl -MDevel::Peek -e 'Dump(%$=[0])'
SV = IV(0x14f025cf0) at 0x14f025d00
  REFCNT = 1
  FLAGS = (PADTMP,IOK,pIOK)
  IV = 1

perl -MDevel::Peek -e 'Dump(%$|[0])'
SV = IV(0x1258256f0) at 0x125825700
  REFCNT = 1
  FLAGS = (PADTMP,IOK,pIOK)
  IV = 4924155560

perl -MDevel::Peek -e 'Dump(%$<[0])'
SV = PVNV(0x12a009810) at 0x104631ea0
  REFCNT = 2147483644
  FLAGS = (IOK,NOK,POK,IsCOW,READONLY,PROTECT,pIOK,pNOK,pPOK)
  IV = 1
  NV = 1
  PV = 0x10447e94f "1" [BOOL PL_Yes]
  CUR = 1
  LEN = 0

perl -MDevel::Peek -e 'Dump(%$>[0])'
SV = PVNV(0x1440097f0) at 0x100bdded0
  REFCNT = 2147483647
  FLAGS = (IOK,NOK,POK,IsCOW,READONLY,PROTECT,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x100a2a951 "" [BOOL PL_No]
  CUR = 0
  LEN = 0

perl -MDevel::Peek -e 'Dump(%$;[0])'
syntax error at -e line 1, near "%$;"

perl -MDevel::Peek -e 'Dump(%$#[0])'  
syntax error at -e line 1, at EOF
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Comment on keys on scalar warnings and lack thereof Download Code

Replies are listed 'Best First'.
Re: keys on scalar warnings and lack thereof (precedence) by LanX (Saint) on May 14, 2026 at 10:21 UTC
`perl -e '$_[0]={1,1,2,2};keys %$_[0]' # no perl -e '$_[0]={1,1,2,2};print ref $_[0]' # wtf HASH` [download] Precedence rules! `$ perl -E'$_[0]={1,1,2,2};say keys %{$_[0]}' # ttf° 21 $` [download] Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery} °) s/what/this/	[reply] [d/l] [select]
Re: keys on scalar warnings and lack thereof by ikegami (Patriarch) on May 14, 2026 at 16:39 UTC
See Mini-Tutorial: Dereferencing Syntax. The curlies in a circumfix dereference can only be omitted if they contain a simple scalar like `$NAME` (or `$BLOCK`), not something more complicated like `$_[0]`. `%$_[0]` should be `%{ $_[0] }` (circumfix notation) or `$_[0]->%*` (postfix notation).	[reply] [d/l] [select]