Unless your machine has an unusually small amount of memory (<1GB) it is quite hard to see why you would be running out of memory on a 400MB file.

There are some anomolies in your code (you calculate $buf_size but never use it; you use & on sub calls which you shouldn't), but nothing that stands out as being the cause of the problem.

You are passing a string containing the whole file into to_ascii() and then copying it; and then passing the modified copy back and then copying it again; which can be alleviated by using references as below. Try that and see how you get on:

#!/bin/perl
# File: get-count.pl
#
# Perl script to locate the trailer record of a Fiserv data file and r
+eturn
# the following information
#
# Usage: 
# perl get-count.pl INPUT_FILE
# 
# Number of records per the trailer
# Number of records based on the file size and record size
# Record size.
sub to_ascii {
    my( $s ) = @_;
    $$s =~ tr[\x40\x5a\x7f\x7b\x5b\x6c\x50\x7d\x4d\x5d\x5c\x4e\x6b\x60
+\x4b\x61\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\x7a\x5e\x4c\x7e\x6e\
+x6f\x7c\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xd1\xd2\xd3\xd4\xd5\xd6\x
+d7\xd8\xd9\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xad\xe0\xbd\x5f\x6d\x79\x8
+1\x82\x83\x84\x85\x86\x87\x88\x89\x91\x92\x93\x94\x95\x96\x97\x98\x99
+\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xc0\x6a\xd0\xa1]
            [\x20\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\
+x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x
+3f\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x5
+0\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60\x61
+\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\
+x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e];
    return;
}

sub file_info {
    my( $file ) = @_;

    open( FP2, "<$file" ) || die return( -1, -1, -1 );
    binmode( FP2 );

    my( $file_size ) = -s $file;
    my($s) = '';

    seek( FP2, -$file_size, 2 );
    read( FP2, $s, $file_size );
    to_ascii( \$s );
    if( $s =~ /999 (\d\d\d\d\d\d\d\d\d)( +$)/ ) {
        my( $rec_size ) = 4 + length( $1 ) + length( $2 );
        my( $rec_count ) = $1 + 0;
        my( $rec_count_calc ) = ( $file_size / ($rec_size) ) - 1;
        return( $rec_count, $rec_size, $rec_count_calc );
    }
    else {
        return(-1, -1, -1);
    }
    close(FP);
} 

foreach $file ( @ARGV ) { 
    my( $rec_count, $rec_size, $rec_count_calc ) = file_info( $file );
    print "File: $file\nRecord Count: $rec_count\nComputed Record Coun
+t: $rec_count_calc\nRecord Size:$rec_size\n";
}
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