Use a regex.

if ($array[0] =~ /^\d+$/) {
    # do something
}
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^ signifies the start of the string
\d means a digit
+ means match 1 or more times
$ signifies the end of the string

Update:
you can also add [+-]? right after ^ if your numbers can have prefixes. Also, see this page: Is it a number?

Update 2:
Yes, there are obvious loopholes, Anonymous Monk, such as decimal numbers (they didn't specify what they were matching exactly. just "numeric values"). See the link above (is it a number?) for more robust solutions.

Here's a snippet of some nice functions from it:

sub is_whole_number { $_[0] =~ /^\d+$/            }
sub is_integer      { $_[0] =~ /^[+-]?\d+$/       }
sub is_float        { $_[0] =~ /^[+-]?\d+\.?\d*$/ }
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Also, use \z if you wanted to "match the actual end of the string and not ignore an optional trailing newline" (perlre)