but then why does length(@a) report 5 when there is a value in $a[37865]? )

Because the string "37866" has five characters in it. You want 0+@a, scalar(@a) or similar to get the number of entries in the array.

Did you think you were proving Perl arrays are sparse? That's not the case. Quite the opposite, Perl usually allocates more memory than needed to allow future growth. 

$ perl -MDevel::Peek -e'$a[37865]=1; Dump \@a,1; push @a,2; Dump \@a,1
+'
SV = RV(0x8176690) at 0x814ed9c
  SV = PVAV(0x8153c64) at 0x814f6a8
    FILL = 37865           <-- Index of last used ele
    MAX = 37865            <-- Index of last allocated ele

SV = RV(0x8176690) at 0x814f684
  SV = PVAV(0x8153c64) at 0x814f6a8
    FILL = 37866           <-- Index of last used ele
    MAX = 65531            <-- Index of last allocated ele

[download]

(Output trimmed to the relevant.)

So we can't compare arrays and hashes without looking at the costs to do operations or their built-in access methods.

Indeed. They each have their strengths.

In reply to Re^2: What makes an array sorted and a hash unsorted? by ikegami
in thread What makes an array sorted and a hash unsorted? by ikegami

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