Using 'A zero-width positive look-ahead assertion' and 'A zero-width positive look-behind assertion.' in the split should do what you want. Read "perldoc perlre".

my $text;
{
    local $/ = undef;
    open my $fh, 'file_name' or die "Damn $!";
    $text = <$fh>;
}

$text =~ tr/\n/ /;

my @arr = split /(?<=[.,;])\s*(?=[A-Z])/, $text;

foreach (@arr) {
    print ':', $_, ":\n";
}
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