This is one way, less conventional:

use strict;
use Data::Dumper;

my %hash;
while (<DATA>) {
    chomp;
    s/\//}{/g;
    eval "\$hash".substr($_,1,length)."} = undef";
}
print Dumper(\%hash);

__DATA__
/root/dir1/file1
/root/dir1/file2
/root/dir1/file3
/root/dir2/file1
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