You're not even limited to positive numbers. 

$ perl -wle'$[ = -3; $_ = "abcdef"; print substr($_,0,1)'
Use of assignment to $[ is deprecated at -e line 1.
d

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You could get weird constructs that caused Perl to crash: 

$ perl -we'$[ = 3; $_ = "abc"; my $x = substr($_,2,1)'
panic: sv_setpvn called with negative strlen at -e line 1.

[download]

I fixed it in 5.10.1 or 5.12.0 when fixing another bug: 

$ perl -we'$[ = 3; $_ = "abc"; my $x = substr($_,2,1)'
Use of assignment to $[ is deprecated at -e line 1.
substr outside of string at -e line 1.

[download]

Keep in mind $[ is slated to be removed from Perl.

In reply to Re^4: how internally $#array is working in Perl by ikegami
in thread how internally $#array is working in Perl by kiruthika.bkite

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