Is that any way in Perl to expand the variable by in $ENV{$variable}?
I exported "a=T" and "T=b" in a shell, and I run the Perl script in which print "$ENV{$a}\n", but nothing is printed. I want to "b" have printed. How should I do it in Perl?
Those environment variables should be chained you say, so
$ENV{ $ENV{a} };
Note: not $a but a, like $ENV{USER} etc. This uses the hash %ENV (see perlvar), which has the current environment, so with keys being names of environment variables.
It is apparently of interest to use a Perl variable (for the shell variable's name†) in %ENV, and not a string literal as above. In that case we need to pass that shell variable, its name or the value, to the Perl program somehow so to have it stored in a variable; can't just use it directly.
Incidentally, one of the ways to pass a variable from shell to Perl is precisely by exporting it, what then makes it available via %ENV. However, it can also be passed as usual, via command line. Assuming the use of a Perl one-liner (common in shell scripts), we have two options for how to pass
As an argument, perl -we'...' "$var", in which case it is available in @ARGV
Via the -s command switch, perl -s -we'...' -- -shv="$var", what sets up $shv variable in the one-liner, with the value $var. The -- mark the start of arguments.
See this post for details, and perhaps this one for another, more involved, example.
† A comment asks how to pass variable's name (string a), not its value ($a). This doesn't seem as the best design to me; if the name of a variable for some reason need be passed around then it makes sense to store that in a variable (var="a") and pass that variable, as above.
But if the idea is indeed to pass the name itself around, then do that instead, so either of
perl -we'...' "a"
perl -we'...' -s -- -shv="a"
The rest is the same and %ENV uses the variable that got assigned the input.
If a full Perl script is used (not a one-liner) then use Getopt::Long to nicely handle arugments.