#! perl -slw
use strict;
use DBI;

my $dbi = DBO->connect( ... );

## Prepare multiple statments for different DBs
my @sth = map{ $dbi->prepare( ... ) } 1 .. $DBS;

## Piped open pre-filters data thru tail and grep -f
my $pid = 
    open TAIL, "tail -f /path/to/the/log | grep -Pf patterns.file |" 
    or die $!;

my @collected; ## Uploading medium sized batches of data is usually quickest

while( <TAIL> ) {
    ## Decide which DB this line is destined for
    my $dbn = m[some selection criteria];

    ## and put it into that batch
    push @{ $collected[ $dbn }, $_; ## subselect if you dont want the whole line

    ## And when that batch reaches the optimum(?) size, upload it
    if( @{ $collected[ $dbn ] } >= 100 ) { ## 100 is a guess

        $sth[ $dbn ]->execute( @{ $collected[ $dbn ] );
        @{ $collected[ $dbn ] } = ();
    }
};

close TAIL;