Poppycock!

#!perl

use strict; use warnings; use 5.010;

use Data::Dumper;

my @list = qw[ Xindex r-index.txt Index.txt notIdx.txt ];
my @file = grep( /index/i , @list );

say Dumper( \@file );

__END__

$VAR1 = [
          'Xindex',
          'QIndex',
          'r-index.txt',
          'Index.txt'
        ];
[download]

See Eliy's reply about grep-in-scalar-context.