Here is a program that a works on the assumption that a file with 4 space tabs will contain some lines that start with 4 spaces while a file with 8 space tabs will not. It skips any Pod in the file.

    #!/usr/bin/perl -wn

    if (not /^=/ .. /^=cut/) {
        die $tab = "4\n" if /^    \S/;
    }

    END {
        print "8\n" unless $tab;
    }
[download]

This is far from fool-proof however. In fact I'd go as far as to say that no method is fool-proof since a file may have been edited by people with different tab/space styles.

--
John.