use strict;
use warnings;

my $file = do {local $/, <DATA>}; #read the file from <DATA>

$file =~ s/\n\;/\;/g; #replace newline followed by semicolon with semicolon

print $file;

output:
--------
asdfadfasdf
asfadfasdf;asfadfasdfas
fasfasdfas;asdfasdfasd
asdfasdfasfd


__DATA__
asdfadfasdf
asfadfasdf
;asfadfasdfas
fasfasdfas
;asdfasdfasd
asdfasdfasfd