$| = 1; # it's important for autoflush to be on
my $counter;
for ( $counter = 0; $counter < 5000; $counter++ )
{
    # ...

    # check, is this the 5th object?
    if ( ( $counter % 5 ) == 0 )
    {
        printf( "%5d\r", $counter );
    }
}
# clear status
print( ( " " x 5 ) . "\n" );
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Using the mod (or %) operator takes the remainder after dividing the given number. So, in this case, taking the remainder after dividing $counter by five is what we want. When the remainder is zero we have every 5th object.

Printing a number with the \r code instructs the terminal to move the cursor to the beginning of the line without overwriting or going to a new line.