$ 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