The problem here is that "\t" is not interpreted by the shell to be a tab character, but just a regular backslash followed by a "t":

> echo "\t"
\t
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Perl also doesn't interpret this input as a tab, because that would make it impossible to have strings with a literal "\t" sequence. \escape sequences are only interpreted in double quoted string literals (and regular expressions).

The way to pass special characters in bash is to use $'\code', ie $'\t' for tab:

> perl -e '$"=shift;print "@ARGV"' $'\t' a b c d e
a       b       c       d       e
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If you use another shell, look in its documentation.

Ofcourse you could also do $" = eval '"'.shift(@ARGV).'"'; but that would complicate matters if anyone needs to pass something that can't be put in double quotes.

update: fixed typo