I agree with hdb that it's safer (and easier) to use modules. Also consider using usermod --shell <new_shell> system command, on Linux, at least.

One way to replace the shell manually:

use strict;
use warnings;

my $new_shell = "/bin/nologin";

# in-place editing with backup
$^I = ".bak";

# diamond operator "<>" reads from command line arguments or STDIN
while ( my $line = <> ) {

    # skip empty lines
    next if $line =~ /^\s*$/;

    # split on ":" and return the 7th field (list slice)
    my $old_shell = ( split ":", $line )[6];

    $line =~ s/$old_shell/$new_shell/;
    print "$line\n";
}
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Update: using explicit variable $line instead of default variable $_ for better readability.