...
unless ( @ARGV == 2 and -d $ARGV[0] and -d $ARGV[1] ) {
    die "Usage:  $0  input/path  output/path\n"
}
my ( $indir, $outdir ) = @ARGV
my @files = glob "$indir/*.html";
die "No html files found in $indir\n" unless ( @files );
...
for my $file ( @files ) {
    ...
    ( my $ofile = $file ) =~ s{$indir}{$outdir};
    open OUT, '>', $ofile or die "$!";
    ...
}