update: Changed the regex to match the data provided by the OP.

my $high_id = 0;
open my $ID_FILE, "<", "path/to/file" or die $!;
while ( <$ID_FILE> ) {
    my ( $id ) = m/:(\d+):/; # extract first number from line
    $high_id = $id if $id > $high_id;
}
print "highest id found = $high_id\n";
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