sub isNumber {
    local( $_ )= @_;
    my( $ok, $val, $base );
    if(  s/h$//i  &&  s/^/0x/
     ||  m/^0x/  ) {
        $ok= m/^0x[\da-f]+$/i;
        $val= hex($_);
        $base= 16;
    } elsif(  s/o$//i  ) {
        $ok= m/^[0-7]+$/;
        $val= oct($_);
        $base= 8;
    } else {
        my $warn;
        local( $SIG{__WARN__} )= sub {
            $warn= $_[0];
        };
        local( $^W )= 1;
        $val= 0 + $_;
        $ok= ! $warn;
        $base= 10;
    }
    $_[1]= $val;
    $_[2]= $base;
    return $ok;
}
my $num= <STDIN>;
my $val;
if(  isNumber( $num, $val, $base )   ) {
    print "Your number's value is $val in base $base.\n";
} else {
    print "Your number is invalid but might be close to $val in base $
+base.\n";
}
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Note that I didn't treat a leading zero as octal and I'm a bit inconsistant with how much whitespace I'll ignore.

Updated: To return the base.