in reply to index of the minimum element of the array

Works for me on a Win7 box with Perl 5.16 and the addition of an output command: 
C:\>perl -E "@a = (1, 5, 7, 9);use List::Util qw( min max ); my $min =
+ min @a;say $min;"
1

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But, note, using "a" (and "b") as variable names is bad practice because they are special to sort. If you get in the habbit of using a meaningful, multicharacter name -- @array or @arr or @ar in this kind of case -- a non-specific example -- you'll avoid puzzles in the future.

NB also: You posted a dupe moments later. Don't. Register and you'll be able to edit if need be...

/me reddens with embarassment: /me replied to the dupe (Substance up to "NB" was identicial) before noticing that it was a dupe.

And even more chagrin: misread the OP, which clearly asks for "index" rather than value.

Come, let us reason together: Spirit of the Monastery

In Section Seekers of Perl Wisdom