There are quite a few ways in which the code you showed can be improved.

• You should use lexical filehandles, or rather in this case directory handles.
• As documented in readdir, you need to prefix the directory name to the filenames returned, for example with catfile from File::Spec.
• readdir will return every entry in the directory, including other directories, which you can filter with -X, and including the special entires . and .., which you can filter e.g. with no_upwards from File::Spec.
• You should check the return value of move for errors.
• I don't quite understand why you want to rename all the files to single letters, but consider this: what happens if readdir happens to return the filenames in a random order, or more filenames than you have letters for? This is why below I use sort and Perl's magic string increment.

#!/usr/bin/env perl
use warnings;
use strict;
use File::Copy qw/move/;
use File::Spec::Functions qw/no_upwards catfile/;

my $dir = "/home/porter/blue";

opendir my $dh, $dir or die "$dir: $!";
my @files = grep {-f} map {catfile $dir, $_}
    sort +no_upwards readdir $dh;
closedir $dh;

my $newname = "a";
for my $file (@files) {
    my $newfile = catfile($dir,$newname);
    print "$file -> $newfile\n"; # Debug
    move $file, $newfile or warn "$file: $!";
    $newname++;
}
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Update: Fixed: "sort no_upwards" was not correct, but the -f test was hiding the issue.