We could adjust the answer given by mr_mischief by pulling the if block out of the inner foreach and using the whole file's size instead of calculating a remainder.

#!/usr/bin/perl

use strict;
use warnings;

my $parts = shift; ### how many parts to split
my @file = @ARGV;  ### the files to split

foreach ( @file ) {

    ### how big should the new file be?
    my $size = (-s) / $parts;

    ### open the input file
    open my $in_fh, $_ or warn "Cannot read $_: $!";
    binmode $in_fh;

    ### for all but the last part, read
    ### the amount of data, then write it to
    ### the appropriate output file.
    for my $part (1 .. $parts - 1) {

        ### read an output file worth of data
        read $in_fh, my $buffer, $size or warn "Read zero bytes from $
+_: $!";

        ### write the output file
        open my $fh, "> $_$part" or warn "Cannot write to $_$part: $!"
+;
        print $fh $buffer;
    }

    # for the last part, read the rest of
    # the file. Buffer will shrink
    # to the actual bytes read.
    read $in_fh, my $buffer, -s or warn "Read zero bytes from $_: $!";
    open my $fh, "> $_$parts" or warn "Cannot write to $_$parts: $!";
    print $fh $buffer;
}

__END__
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The same weeknesses exist for memory usage. There could be better checks for file existence, too.