Most of your time is being spent building an 8-char string from 8 random bytes, then copying that to a 16-byte string and adding 8 more random bytes; then copying that to a 32-byte string and adding 16 more random bytes; then copying that to a 64-byte string ...

Try this version:

#! perl -slw
use strict;

my @chars = ('a'..'z', 'A'..'Z',0..9, qw[- _ ! & ? = ] );
sub genFile {
    my( $fh, $mb ) = @_;
    my $buf = chr(0) x 1024**2;
    for( 1 .. $mb ) {
        substr $buf, $_, 1, $chars[ rand @chars ] for 0 .. 1024**2-1;
        print $fh $buf;
    }
}
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