{   # note the {} brackets. This concerns the scope of the next line.
    local $/;  # sets $/ = undef for this block only.
    open FH, $infile or die $!;
    $_ = <FH>; # Reads the entire file into memory.
    close FH or die $!;
    @_ = m/(some-regex-for-the-block)/gm;
}

# now @_ contains an array of matches for the regex.
# write each one to a different file:
my $filenum = 0;
for( @_ )
{
    ++$filenum;
    open FH, ">MATCH$filenum" or die "Failed to open MATCH$filenum, $!";
    print FH $_, $/;
    close FH or die $!;
}