This isn't going to work. $1 isn't defined as no capture has taken place.

Try this.

# For foo.htm => foo.htm.tmp
opendir( HTMLSTORIES, "$html_dir") || die "HTML files do not exist: $!
+";
@FigureArray = grep{/\.htm$/} readdir ( HTMLSTORIES );
foreach $figfile (@FigureArray) {
    copy ($figfile, "$figfile.tmp") or die "Can not make temporary cop
+y of file [$figfile]: $!";
}
[download]

BTW, make copies of the original before overwriting them. The last thing you want to do is blow them all away and realize you don't have backups.